Why go to all the bother of replacing the outlets with 2 prong? Why not just tape over the third prong and paint it to match the outlet and there! You have a legitimate 2 prong outlet. Would it be any safer? You can use an adapter just the same. (That was just a little thought experiment). Essentially, a 3 prong outlet with the third prong ungrounded is already just a 2 prong outlet - you just may not realize it (it should be marked "no ground").
So does the lack of a seat belt in a car pose a hazard? Well, so long as you don't get into an accident then it's perfectly safe. Not having the third prong electrically simply provides no protection should something go wrong, like a dangerous short. It doesn't pose a hazard in and of itself, anymore than a lack of a seat belt does in a car.
So, do you want to provide the protection that's missing? Chances are nothing bad enough will go wrong such that you would need the protection, just like chances are you won't get into a bad enough car accident to need a seat belt, and some people don't even wear their seat belt. But suppose you want the protection, what can you do? The two best thing's you can do is to either install wiring (install the seat belt) to ground the third prong on the outlet or install a special kind of outlet (air bag) that senses when something bad is happening and quickly shuts the outlet off. That's called a GFCI outlet (Ground Fault Circuit Interrupter). They are available at any hardware store for about $15. Get the kind that automatically go dead if they fail to function normally.
Keep in mind that your outlets may already be protected by a GFCI device, either the circuit breaker in the main service panel is a special GFCI breaker that protects the whole circuit or else the first outlet on the circuit is a GFCI and all of the other outlets downstream are protected on account of that one. Verify this first before going to buy any GFCIs.