I routinely freeze software instruments as soon as I feel they're "done". It's a habit going back to the days of much less-powerful computers, when freezing was often unavoidable. Although rarely
necessary anymore, it still has the benefit of drastically speeding up project load times.
"Quick Freeze" should probably be called "Re-freeze". It just allows you to un-do a quick unfreeze.
At risk of speaking for Jonesey, I would guess it's because freezing synths offers the greatest benefit. The only way freezing other tracks would be of help is to freeze their effects, and most of us prefer to keep most effects live for mixing.