I think most RPGs end up offering strange incentives that don't exactly make sense from a "real life" narrative standpoint, but an incentive to maximize exploration in a first person game seems pretty benign. Players want to see all the content your game has to offer anyway. What they don't want is to have to kill/neutralize all the enemies in a certain way (although they might want to neutralize them all in some way).
Eh. The more I game, the more I realize that
not seeing every last air duct and rooftop is much more satisfying than having that feeling, "I'm ready to finish the level, but first I should scour for carrots."
Ruhfuss is right - objective xp is the way to go. If I want to see everything for its own sake, I'll go look at it for its own sake.
When I see a tool shed and think, "I bet that tool shed is very detailed inside, but there's nothing in there I need, so I'm not going to look. My time is better spent elsewhere",
that's when I know I'm in a truly fleshed out and well realized game world. If I think, "I better open it in case I get xp",
the devs fucked up.
If the goal really is to incentivize "nook and cranny gameplay", Thief did it much better by offering optional objectives like "steal 5000 gp worth of valuables". Gives the player a much more natural motivation than "another empty room! congrats 500xp".
I still remember the first time in Thief when I broke into some place just because it was there ... and it was empty. It was just a realistic place, it made sense it was there, and it made sense that there was no game reason to go there. That moment changed the way I thought about level design, in a very good way.