For lack of a better skill, I too have used "spot" for scents.
Most of the time, however, I rely on skills related to the scents. For something common like "smoke" spot is good enough.
If the characters are walking on a footbridge over a huge pool of acid - they're ALL going to smell something, but I make them make an alchemy check to identify what the heck it is that's burning their nostrils.
I think the game assumes that since humans (and similar races) don't have highly developed senses of smells, then smells need to be pretty powerful to be detected. Basically, either the smell is strong enough to be detected, or it isn't... there's no check involved to see if you can successfully "smell" something. I don't use scent that often, but when I do, it's usually a matter of the characters trying to identify what they're smelling, than the fact that they smell something in the first place.