This is very true, and the best thing to do often comes down to exactly what sort of engine/lighting setup you're using. Sure you can break some stuff down mathematically and have all the theory you like, but when it comes to practical application you just need to do what looks best. =)
A lot of people just multiply their AO on their spec maps rather than bake the spec with a lighting set-up. Ideally a spec map is a representation of the reflectivity values of a surface. And thats pretty much how it is in high end graphics engines and offline rendering. People put their AO in there to help bring out areas…
Baking specular is to get a base, further editing will be needed to add the higher and lower values within the different material areas like scratches, wet areas, dirty areas ect, do this in photoshop ect. You can bake one without without using lights. If you want to use lights like pior did, it's just to give a bit more…