Lucas Giolito has a golden arm. It's golden because he has all the talent in the world, enough so that Baseball America would make him the #21 prospect after seeing just 40 innings of him in the minors. Baseball Prospectus basically says he has Hall of Fame talent (at least from what they can see right now) and rank him I believe #13 (but don't hold me to that).
It's also golden because pure gold is pretty damn soft and he's already had Tommy John surgery done for the wear and tear put on his arm facing high school talent. He's only got 38 IP so far in the minors which tells us basically nothing about the long term health of that arm.
Anyway, the point is the Nats drafted him for the great arm and ignored the realistic potential that the kid might not heal right (signs look good that he has) or get injured again (jury will be out for a while on that) while other teams were more cautious. They did a similar thing in drafting Rendon (though I'd really argue he dropped about as far as he was going to go, the Nats were just in the right spot at the right time - I'm digressing again!). Why?
#1 Kershaw 6.5 WAR
#10 Darvish 5.0 WAR
#28 Zimmermann 3.6 WAR
The difference between Yu Darvish, who put up a 2.83 ERA in the AL pitching in Texas, and Kershaw, was the same as the difference between Darvish and ZNN, who had a very good year no doubt but one that, let's admit, petered out at the end. If you don't see ZNN to Darvish as a big difference, (I don't know maybe you don't follow the AL) what about ZNN to Cliff Lee (5.1)? It makes the same point. ZNN is a very good pitcher. What would you say, a very good #2? Darvish/Lee are among the best #1s in the majors. The difference between a very good #2 and the general group of best #1s, was the difference between that group of best #1s and Kershaw. That's how much better he was than anyone.
The same thing can be seen with hitters
#2 McCutchen 8.2
#12 Cano 6.0
# 33 Adam Jones 4.2
Cano, who hits for average, hits for power, and still plays a decent 2nd base, is more valuable than Adam Jones who hits for power but only a good average and is kind of out of position in center. The difference between them is the difference between Cano and McCutheon, who basically does everything right.
Elite, best in the majors, talent separates itself even from the rest of the best. If you can manage to get that kind of talent on your team while it is young and relatively cheap?Your job just got that much easier.*
So the Nats gamble. Prospects often don't work out so they go big every time hoping for the straight flush to show up. It may usually fail, causing organizational depth problems, but if it succeeds even once every 3-4 years, it can be worth the risk.
*"But Harper!" You say. "The Angels have Mike Trout, the elitist of elite talent (10.4 WAR) and didn't even make the playoffs!" Here's a couple fun facts. The Angels team ERA was 11th in the AL. That's bad. The Angels offense with Mike Trout ranks 3rd in average, 3rd in OBP, 5th in SLG. That's good. hence the around .500 record. The Angles offense without Mike Trout would rank 6/7th, 8th, and 9th in those fields respectively. That's below average. Not to mention he plays good CF and accounts for 40% of the Angels SBs (82.5% success rate). Without Trout the Angels would have likely won fewer than 70 games last year. It's makes the job easier, but it doesn't make it easy.