I see both sides. Promoters love it because having blue chip drivers in the Truck or Nationwide races will put butts in seats. Drivers do it to get more seat time or to get their driving fix.
Other drivers probably hate it because it's not entirely fair. In it's essence the Truck and Nationwide series were designed as developmental series. So having accomplished Cup drivers drop down is kinda like Tiger Woods popping into Q-school just because he wanted to get another round of golf in.
Personally, I don't like it. When I watch a Nationwide race, I'm far more interested in the Nationwide regular who finishes highest. And I LOVE when a regular wins the race. When someone like Kyle or Harvick or Edwards wins a race, it's meaningless in my eyes. And when they win the Nationwide Championship like has been the case in the last 3 seasons, it makes it even that much more unfair. It has gotten to the point where the Nationwide race is merely the Saturday version of the Cup race. Watch the race Saturday. Then tune in Sunday for the second half.