In September 2003 I was building a belt grinder. I had a DC motor and I asked if I should use a variable speed controller. RCM'ers said that I should and I did. They were right - I use the speed adjustment all the time and I hardly ever use top speed (6000 fpm).
Also, there was discussion about it being an open motor, with brushes no less. Well, this was/is a treadmill motor from the dump and easily replaced, so I didn't worry about grinding dust getting into it. It's been 3 years and it's been fine. Out of curiosity I went down today and blew it out with compressed air. There *was* quite a bit of dust in there but it hasn't affected how it has worked. YMMV, of course.
Thanks RCM, Bob