I mean, from Disney's viewpoint, better to make $30 off somebody than $0, right? If $87 is too much for someone (especially someone who is both limited in budget and time), but $30-40 is within reach, why not have them spend that money in Disneyland, rather than at Knott's or somewhere else?
Would it solve the crowding issue? I don't know. Maybe, maybe not. That's another issue, of course. If capacity is reached, they can suspend sales of ticket books at the Main Gate until the capacity issue subsides (in-park ticket sales could still continue, though).
I just find it a bit peculiar that so many are naysaying this idea, when there are already 4 variations of the Annual Pass, three of which involve blockout dates (which necessarily make them limited access passes). This is just transplanting the same sort of idea and its inherent flexibility to the infrequent or one-time Park visitor, as well as those of us who have to manage more limited budgets.