It's actually pretty smart to play with play chips if you have a low bankroll. Let's say you're playing on Full Tilt. There are "high stakes" tables where you can sit with 5mil+. If the going rate for 1mil play chips is $5, then this is like sitting with $25. Except.... no rake
Of course when you sell them back it will be for less per million, but still I think it's smarter than grinding out 10nl and below.