If I use 800 minutes a month, I'll be giving up 200 minutes with T-Mobile, or at AT&T for the same price, I'd have 350 minutes of overages, a huge bill, and no rollover minutes to carry over.
If you always use 800 minutes (800, 800, 800, 800...) then sure, you're right. But if it looks more like this (200, 350, 450, 300, 300, 700, 300, 550) then rollover works. Obviously, you have to look at your own usage to see which style works better, but most individual plans tend toward fairly low use (relative to the montly limit) with spikes every few months. These people are "wasting money" when the plan doesn't have rollover.
It all comes down to variance. Variance seems to be in the 100-200 minutes range, which puts even you in danger of extra charges with no rollover.