Pluses:
-Allows superstars who were injured during their trials or weren't picked by a corrupt federation to still make Worlds.
-Ties the season together. Now each Diamond League meet is connected to Worlds, and the meets aren't completely exhibitions that have no bearing on any other race.
-The excitement of the last few Diamond League meets as athletes who aren't in Worlds get desperate for a spot.
Minuses:
-It takes away from the "do or die" nature of the US championship meet. This applies more in the sprints, and some field events, because most U.S. men distance runners aren't threats to win a Diamond League meet (although Lagat is, and on the women's side several could do it).
-It allows more (African) countries to have 4 competitors rather than 3, so it will bump down the finishing places of US distance runners even more.
-What about the events that aren't contested at Diamond League meets? It seems odd that athletes in some events will get multiple chances to qualify, but those in other events don't have that option.
Overall, I say the pluses outweigh the minuses. It can't hurt to change things up and see where the chips fall.