The main problem people have with
It's a valid question. Lobaton at 28 had a decent season at starter/back-up for the Rays last year but previously had put together one season (2011 in AAA) that even remotely pegged him to be a decent starter. They say catchers develop late and perhaps he did. Good minor league 2011, standard pretty bad back-up catcher in 2012, and good part-time starter in 2013. The trend is there if you want to see it. But 28 isn't just late, it's peaking and chances are he's given most of what he can.
Karns on the other hand is a true power arm, who racked up impressive strikeout and ERA numbers in the low minors in 2011 and 2012. The downside, as it is with most power arms, is that walks have always been an issue for him though. In AA last year he wasn't as impressive. He still struck guys out but he really gave up the long ball at a much higher rate. That was certainly evident in his brief major league stint where he gave up 5 HRs in 12 innings pitched. That's "OMG PITCHING MACHINE HR DERBY!" levels. At an older age 26* he's also on the older side for a prospect.
Still Karns has that potential, why deal? Because the same reason you applied to Jose Lobaton. If all goes right Nate Karns will also barely be used.
If all goes right Detwiler is healthy, takes the #5 spot and Taylor Jordan, 25 and with a better major league audition, takes the role of next-up. What about 2015? The only thing that changes in this perfect world is that the much younger AJ Cole would take his place in line, likely ahead of Jordan. And in 2016 when ZNN and Fister might be gone? If all goes right here's Lucas Giolito stepping up while Karns, all the while doing well in AAA (it is an "everything goes right" scenario) slides from "maybe 2nd in line" in 2014, to "time to make him a reliever" in 2016.
"But everything may not go right!" you say. EXACTLY. That's why you want Lobaton! Ramos might get injured. Chris Snyder is probably done.
Karns does have potential, but at only a year and a half younger than Zimmermann and Detwiler, and older than Strasburg he's rapidly aging out of the range where you might expect big things from him. A mediocre 2014 paired with a couple guys doing well would virtually end the chances of Karns being a starter for the Nats.
The deal is a good back-up catcher now for a guy who might be a back-end starter for a couple years. Its a potentially very small gain for 2014 for a probably at best modest gain in some future time frame. The future still matters. That's why you don't deal a Cole or Giolito for Lobaton. But 2014 matters too, which is why you do deal a Karns.
*For my purposes I think like this - an "old" age for a prospect is someone who turns that age at the back end of a season. an "older" age is someone who turns that age between seasons end and the new year. A "young" age is someone who turns that age during the season in question. This helps differentiate between the 25 year old who will actually turn 26 in say August, and the 24 year old who turns 25 in May. For stat keeping they are both 25, but there is a difference in how you'd evaluate the two. Relevant for me between ages...say 22 and 28. Before 22 you are pretty much just young. After 28 you are pretty much just old.