Playing in a freeroll is always plus EV, and pro players love +EV)))
So it would be quite logical for them to play in freerolls.
On the other hand, the prize pool in freerolls is pretty much always quite low, while number of entrants is quite big. Hence, as profitable it might be in terms of EV, no pro that has a
bankroll will bother playing freerolls.
I think freerolls are meant to be for player who are willing to try playing poker seriously, but do not have spare cash, or simply do not want to risk money while trying. Once a player generates any kind of bankroll, he usually stops playing freerolls as it is much more efficient to play tournaments where prize pool is generated through buy-ins and thus can reach tremendous amounts.
Anyways, even if lots of pro players are participating in freerolls I do not think that it is bad. Personally, I enjoy playing against strong opponents rather than fish.