I have been riding and racing hardtails all my life (I'm 30 now). Over the past three years, I've been racing DH with a Balfa MinuteMan w/ a SuperT. I feel like I'm a pretty smooth rider because o fthis time spent "paying my dues". This year, I moved up to the open Expert class, and I'm getting absolutely killed trying to keep up with the boys on their M1s and BB7s.
I've finally taken the plunge and ordered a BigHit Comp (my SuperT will be going on it). This will be my first duallie, and I think I'm gonna love it.
Anyway, all that to say that I think that spending time on a HT WILL indeed make you a better rider. I'm looking forward to going faster than ever on the duallie, but I don't think I'd be as comfy on it without all the time spent on the Minuteman and other HTs I've owned. I do plan to get another one someday, but I can only afford one bike right now.
BTW, my MM frame is for sale. PM me for details! :)