Every other ally that lets you explore requires you to discard them. He lets you get an extra explore every turn and not loose any cards in your deck (because you return him to the TOP of your deck). Sure, you run a 1 in 12 risk of loosing all the cards in your hand (which is what you will do if you come across the sandpoint devil), but if you get lucky, you could potentially have double the explores.
He does allow you to encounter non-monsters, right? I don't have the card in front of me right now.
Yeah, I think Shalelu is by far a superior scouter/explorer. You don't get her back each turn since she recharges, but not summoning the sandoint devil, and having the option to encounter what you scout is great.
I had great luck with him for a while. I want to say I've used him two whole games and never summoned the devil once. With him, you essentially get a "free explore" every turn. It's not a FULL explore, but it's nothing to sneeze at either. If you are playing a hardy type (four card hand size), and you manage to play him four times without summoning the sandpoint devil, then it's already more efficient than using four blessings. The way I see it, if the SPD gets summoned, you'll loose a maximum of three cards in your hand (only three because he's already on the top of your deck). Once you you can play him (Hand size -1) times per game, you've made it more efficient than using allies/blessings.