First off, there's no such thing as a stupid question. The only stupid question is the one never asked.
Macro lenses give you the ability to create very close-up pictures of objects. I'm the proud owner of a Canon 20D and I happen to have a Canon EF 100/2.8 Macro USM lense. While I wouldn't say the lense is useless for photographing your models my personal experience is that it's hardly necessary to own one if your only use for such a macro lense would be to photograph your models. The only advantage you'll have is the abbility to sit away 3 feet and still be able to photograph a 1/35 figure head and filling the complete image with it. That up close is what you'll get.
A decent macro lense isn't cheap so I would recommend purchasing different hardware to enhance your photography experience. Like a remote control cable for your camera and a decent tripod. These are badly needed if you wish to create sharp detailed pictures of your models.
With your 17-85 (is it the kit lense?) you can get close enough to your model and photograph all the detail you wish.
Hopes this helps.
PS: A few examples I made a few weeks back with my Canon 20D and Canon EF 17-40/4.0 L USM 77mm. All pics where given a Photoshop treatment to enhance color and sharpness.
The following pics are made with my 100mm macro.