Before I get into this I would just like to say - Anthony Rendon would be a fine All-Star. Anyway you look at him, fancy stats or traditional, 2B or 3B, you can make a good argument that he should be on the squad. Of course when it comes to the All-Star team, good arguments aren't always enough. The one-per-team rule (which I wholeheartedly agree with), the veteran/star skewing perceptions of the fans and players, and the manager's desire to pick his own players all work to make only the most standout players sure things. But good arguments are worth putting out there and fun to discuss, which is why I'm dismayed that perhaps the worst argument for why Rendon is a better player than the ones selected, is the one most frequently cited.
By this I mean the focus on Rendon's Runs scored and RBI totals. As far as ranking the absolute talents of a player, these are very questionable stats to use* because they rely on two factors, the talent of the player himself and the talent of those around him. For example a player that gets on base at a .400 clip would be likely to score a lot of runs. But if he's followed in the lineup by Danny Espinosa and the pitcher he's going to score a lot fewer times that you'd think. Conversely .320 OBP guys shouldn't score that many runs, but put him ahead of Miggy and Trout and he's going to score more than you'd expect.
One way to see if Rendon is outperforming in these categories because of things beyond his control is to compare his rankings. OPS correlates very well with runs scored. This should be obvious. You get on base, you have a good chance to score. You get a XBH, you have a better chance to score. OPS factors in both and the correlation is high. Rendon ranks 4th in the NL in Runs Scored. He ranks 20th in OPS.
It's easy to see why there's this discrepency though. Rendon is followed in the line-up by two hitters having good years in Werth and LaRoche.
Werth .279 / .364 / .421, .321 / .427 / .556 with RISP
LaRoche .289 / .393 / .470, .302 / .467 / .476 with RISP
They are hitting well in general and better with men in scoring position. (After a total non-clutch year, Werth is doing the opposite this year. .315 / .402 / .490 with men on, .251 / .333 / .369 with no one on) Not every player is going to have this following him.
RBI is slightly different in its correlation. Factoring in OBP is not great for RBI because walks rarely drive in runs. Instead you are better looking at SLG alone (other fancier stats would be better but SLG works well enough for this - there's not that much variation for Rendon). Rendon ranks 8th in RBI and 17th in SLG. He's outperforming here too.
But why? This one is harder to make sense of. The easiest answer would be that he's hitting better with RISP but that's not the case. He's hitting ok, but a bit worse than overall. (.284 / .340 / .491 to .259 / .330 / .482 with RISP). Hitting 2nd helps as it means more at bats, but that alone can't explain it. As you know Span is not particularly adept at getting on base and before him would be the 8th spot and the pitcher. Despite all that though Rendon is driving in more runs than expected and is seeing a fair amount of opportunities with RISP (20th in the NL). So I dug further and here is what I found.
(1) Rendon is hitting well and hitting a lot of HRs with men on. RISP hits matter but so do hits with men on first and Rendon is killing it there. With men just on first he's slugging an amazing .702. Even though those only account for roughly 15% of his PAs, his hit 2 of his 5 triples (40%) and 4 of his 13 homers (30%) in these instances. Doesn't matter if they aren't in scoring position if you can hit well enough to knock them in from first. This is all him.
(2) The Nats 8th place hitters haven't been all that bad. No a .235 / 304 / 349 line isn't good but compared to the rest of the league this combined OPS is 3rd out of all teams. I assumed the Nats 8th place hitters would have been among the worst. It isn't helping Rendon but it also isn't hurting Rendon as much as I thought,
(3) Matt Williams loves to bunt with his pitchers. The Nats 9th place spot is 2nd in sacrifice hits with 28 and when we look at pitchers specifically they lead the NL (with 28 - not sure how Cincy pulled off 5 SH from non-pitchers in the 9th spot in their lineup but there you go). Obviously this gives Rendon more shots with a RISP assuming they are still there after Span gets his turn.
(4) Span is really good at getting himself in scoring position for the players behind him - If we look simply at the number of times Span has put himself in scoring position (and not run himself out of it) we find that Span ranks very high in the NL. Third in fact. I estimate this by using this formula 2B+3B+SB-CS. Obviously that's imperfect (some of those SBs could be of third, you can hit a double and get thrown out trying to stretch it) but I think it works well enough. The only players who have done this more times are Dee Gordon and Billy Hamilton. This is a big reason why Rendon is seeing a lot of PAs with RISP**.
Rendon is good enough to be an All-Star, and he is better than some of the players selected. But let's not say he's better because of his high RBI and R totals, because a big part of that is the team around him. There are better ways of saying it.
*I actually don't mind using these stats at all for determining All-Star appearances or awards, etc. That's because I frame these things as asking the public "who had the better year" as opposed to "who is the better player". The former includes things like luck and the team around you and I think it's totally fair because THEY ARE JUST SILLY AWARDS. But I do have issue if you are going to use these stats to say the latter, which is how I interpret most of the Rendon push.
**Note: Please do not confuse this with "Span is really good". Part of
this is that Span is incapable of hitting home runs and hitting home
runs is the best thing you can do at the plate. Another part of this is
that OBP matters less in this stat so Span's failing there barely
matters here. We just told you OPS correlates well with runs scored. I'm sure better than my "get in scoring position" stat. Span's OPS is blah. Plus, we just mentioned how well Rendon has been hitting with
men on 1st, and he's a natural doubles hitter. Imagine if Span was
getting on base like a good leadoff hitter should.***
***Remember, I'm also not saying Span is bad though. Just meh at the
plate. Fields well, runs well, worth starting, just not batting first.
Although you can probably also figure out how that really doesn't matter too much. Put Werth first and maybe he scores a couple more runs because he gets on base a lot and hits more home runs and now he has more PAs, and he drives in a couple more of those 8th and 9th guys. But there's a tradeoff from however else the lineup is adjusted.Overall it's a small difference which is why optimal lineup for a season is not a deal breaker. But you do want to see your team giving itself the absolute best chance to win and in a single game do you want Span up one more time or LaRoche or Desmond?