I almost always hire a make-up artist for my shoots (some have been working with me nearly 10 years), and they are totally worth it.
This is important because I work with lots of amateur and newbie models, and most of them really don't know how to professionally apply make-up, or how to achieve various looks (I shoot some girls 20 times or more, so having different looks is important to me).
A side benefit is that the girls feel really pampered (they don't know that I hire the MUA's as much for my benefit as theirs).
I have to admit that it took awhile to get used to waiting about an hour or more per model for hair and make-up. This is one reason why I never pay models by the hour, since they often spend more time doing hair, make-up and wardrobe, than they do shooting - plus I usually hire more than one model a day (to save on location and crew fees, which are usually day rates), so while one model is shooting, another is in the make-up chair getting prepped.
I <3 my MUAs...
ADG