They've always been pretty low value. Here's a quick list of guys taken in the first round of the last five drafts who I'd say are currently better than the average MLS player. I came up with 8/105, with maybe another 20 or so in competent starter range. Some of the younger guys might get there eventually, but still, you're looking at something like a 2/3 chance that a first round pick will be basically worthless.
2014 (19 picks): Steve Birnbaum
2015 (21 picks): Cyle Larin, Tim Parker, Cristian Roldan
2016 (20 picks): Jack Harrison, Keegan Rosenberry
2017 (22 picks): Lalas Abubakar, Julian Gressel
2018 (23 picks): none yet
I'd like to add...
2014: Andre Blake, BEN SWEAT
2015:
2016: Brandon Vincent
2017:
2018:
Yeah, the SuperDraft is becoming more meaningless.