For me it's more about internal consistency than pure spell power. For example, Create Pit is something I think logically would ignore SR, after all you're not actually targeting a creature with it. Furthermore, if it was subject to SR how would you adjudicate it? A creature whose SR you fail to penetrate just floats above the pit?
I don't disagree with that, but if we decide the spell has to be built that way, maybe it isn't a 2nd level spell.
A level 3 version of Create Pit which somehow allowed SR would still see heavy rotation in my prepared spell list -- so maybe it's actually something like a level 4 spell. Glitterdust, same thing. It's when a bunch of spells are among the best for its level without SR being considered AND also happen to ignore SR that the whole balance of SR just falls apart.