The ability to get a preview of an area by helping someone else complete it is a really fun feature of the game. Particularly as an Heir of the Sun, getting summoned is really easy and you can often be very useful - there's a real strength in numbers effect.
The only issue I have with summons for exploration is this: everyone sucks at Dark Souls! No matter how hard I try to keep them alive I've never, ever made it to 15 minutes helping a given person and so never actually gained a reward from it. For that, I guess you have to stick to boss co-op.
