Exactly. There are people that do take other non-SR modes (even ARAM and TT have their share of players that play those modes with the mindset to win) seriously. The way you said that Dominion should be used to test out builds in this manner can apply the other way. What if someone who claims to be a kickass Kassadin player on Dominion (who is actually very OP in that mode), wants to first pick him in ranked SR or draft SR, despite not playing a game in that mode with him? What makes it fair that people can't test things in SR (normals of course), but testing in Dom, TT, or ARAM is?
You would be surprised how people underestimate having a balanced team in other game modes. Having 5 squishy damage dealers may mean that you can assassinate enemies out of position, but will melt in teamfights. Also, the bottom laner in Dom is akin to the top laner in SR, where it can make or break a game.
Being behind or out of place with a build on Dom is punishing itself, as aside from one-sided stomps, the gold counts among all the players are close to even with each other.