Once I got used to the timing of running close to the wall and hitting Triangle just as I reached the wall, I had little problem using cover in MGO. The problem really is that:
1. Movement on the wall is too slow to be useful in reactionary combat situations.
2. Peeking out takes too much time, and you can't peek over, only to the sides.
However, if you could snap to the wall and move very quickly, and peek in and out in an instant, I'm afraid that you'd be able to just peek out, go for a headshot, and duck back a little too quickly. In MGO if you have a good bead on someone, you can headshot them extremely quickly and kill them instantly. With this slower cover, you're exposed for a moment while using it, so it's not too powerful, but I'd be afraid if you could snap in and out in half a second.
Also I think blindfire has no place in MGO, although GTA4 and Gears is fine.
In single player, your camo goes up on the wall, and it's usually a bit more slower paced, so you can take your time to line up shots, wait for the enemy to turn their back, pop out and shoot. In SP, you're probably going to benefit a lot when shooting from cover, more so than MGS3, because your camo index doesn't go to zero anymore when you're trying to hide, there's no more peek from FPV, and you're pretty well protected when you just peek out a little bit in SP.
My theories, anyway.