Everyone carries a 30 degree because that's what the stores stock. You have to custom order to get lower.
I have heard the reason for the offset is less wind resistance for the blade that's in the air, and that it's a hold over from ocean/lake kayaking. Wind resistance doesn't really concern me 99% of the time in a whitewater kayak (the paddle out of Westwater or something similar being the rare exception). I've never heard of any other benefit of the 30 degree offset.
A 15 degree or 0 degree supposedly makes playboating moves easier, makes your offside roll easier, and is more ergonomic (less twisting of your wrists). I say supposedly because I haven't had a chance to use one, but all of that makes sense to me.
My next paddle will be a 0 degree.