The long and short of it is basically, yes. You likely need a makeup artist. Why? Well, there are a few reasons. The most important of which is that you will be much happier with the end result. Some things to consider:
A professional hair and makeup artist does this for a living. They understand the specific makeup needs associated with HD photography.
The HMU artist is a third person on hand to be watchful of your wardrobe, your hair and YOU as the shoot progresses. The photographers primary job is to give you direction, make you at ease, and compose the perfect shot. That's much more than you'd think and the photographer can miss stray hairs, or creases in your shirt as they are focusing on making you comfortable and learning how to direct you for the best result. The hair and makeup artist is there to be sure you look your very best at all times!
Having your hair and makeup done by a professional can make you feel much more at ease. It allows you the opportunity to focus on the job at hand - acting for the camera.
If you are looking to do a few drastic (or even not so drastic) look changes, the HMU is going to help be sure that each look is unique and perfect.
Chances are you'll be much happier with the results with someone professional there to make sure you look your best.
In the end, if you have a professional makeup artist you will end up with many more photos that you LOVE from your session than without. No one has ever regretted having their hair and makeup done by a professional on set...but certainly there have been people who wish they had made that investment!