FYI, you can add resistance to your rollers and convert them to free motion if you're willing to spend a little time and not much money. There are several threads on DIY free motion rollers. Some are very elaborate while others are pretty simple.

To answer your question, the free motion keeps the rollers under you when you ride out of saddle. You can ride out of saddle on normal rollers too, it's just easier with the free motion rollers. They also feel a little more natural.