I tend to think you lose some of the 'rush' if you play only for play money ... keep in mind freerolls are different because you still do win real money . As to whether you could only play freerolls; well I think they are not as exciting on a continuous basis as regular buy-ins are due to the large number of players who are too reckless in them ; myself included at times !!
Use the freerolls to try all different styles of play, experiment and find what works for you and then use what you've learned in some smaller buy-in tournies.......
Of course, this is just my opinion......