Waste of time and the chance that at any moment you will deposit your own money as you going to think that you already understand the game and you find the best strategy to win. It's a worse thing that will come up in your mind instead of bringing you away from gambling. It will keep you near, and the lust and desire will overflow inside you.
I believe gambling with fake money is a waste of time to gambler who aims to get profit for every spin. But for the people who just wanted to look for entertainment and check the mechanics of the casino. I believe it is not a waste of time since we are able to see the mechanics of the game without spending a dime.
Best to find other alternatives to keep yourself busy, much better than using fake money and spend time from playing using fake money.
You can't advise that to people who wanted to play slots for entertainment and don't want to spend a single cent.