Well I think Mitch uses what ever he feels will get the job done the way he sees it..
But it also depends on what you mean by illustration , to me any art that illustrates something in the real world is an illustration painting .
So if you are painting a portrait of a car you have illustrated the car .
But it is mainly finding what paint works best for you , Lots of folks love and use E'tac , Me , I fight with it every time I used it so I no longer use it. But it is good to try different paints to see which fits you and your style of painting.
Certainly, also depends on your surface to be painted, and as micron said what works for you, you wil find god and bad in all paint thats for sure...I started off using wicked and loved it but thats all I knew, have since used illustration paints and love them! But I find I use more of it as it didn't require the same amount of reduction as wicked to get it to spray nicely.....so many paints, so much choice, so many benefits, so little money garrrgghhh!