An interesting question; particularly since the poll results are so even; that makes it a great topic of conversation.
For myself; I agree with Peter: define 'limits'. There are two; which has of course already been stated; a person's percieved limit; where he feels he can't go farther, and his actual limit, where he actually can't. I've yet to see a single Aikido class that took anyone anywhere close to their actual limit; while the percieved has been crossed many times.
I also agree with Si Wilson; it depends on the student. In our dojo, for instance, there are a few - myself included - who learn best when under pressure; who like
to push the boundaries of what we can do. As I've said before, this gets me into trouble sometimes when I try something and wind up over my head. Still, that's how I learn. There are others in the dojo whom I would never push, even if it would ultimtely help them, for this reason: For an instructor to take a student beyond his percieved limits, he must use a certain amount of aggressive tactics. However well-meaning, supportive and lighthearted, these tactics are still aggressive, and some students simply either don't want that or aren't willing to deal with it - Aikido is as relaxing to them as it is stimulating to me. Attempting to push these folks would spoil their experience, if not drive them away entirely, IMO.