Play money games can actually help you and your game. Not all of us are fortunate enough to be able to afford real money
I think the trick with play money games is to somehow get as far above the min buy-in tables as possible. The play at the cheapest play tables is horrendous.
Using FullTilt SnG's as an example;
games are hard. This is where beginners play, and drunkards. Either will play anything and this is not even as good as a kitchen table game. HOWEVER you have to feel comfortable at these and they are beatable.
games show some improvement. In order to get into these you have to have shown an ability to do well in the 200 games. These 2000 games are closer to what poker is, and play approximately like a freeroll after the first half have been eliminated. This is a good place (not the best) to practice particulars in your game.
games are where good pretty solid poker is played. This is where you can make that million playchip goal we all have. These play similar to a $10 or $25NL real money game. People at this level are still playing play chips, but they are very serious about 'their' play chips.
Curiously, the 100K
games are not as solid as the 10K games, but may be more like real honest to god poker. At the 100K level the play is solid and all styles appear. Few beginners have gotten lucky enough to afford the buy-in to even play in these. Once the LAGs are gone these play very solid.
Personally I have played all these levels, got close enough to my mil goal, and seldom play them any more. But when I need to change something about my play, it is at the 10K level where I can feel confidant that the play will not revolt me.
So the answer to your question is "it depends". It will depend on your approach to the game.