I agree with Doug. Here's why...
Basic strumming is done with the index finger and the thumb doesn't normally go below the strings at all. If you're using your thumb for the upstroke in 4/4 time though, then you're wasting enough energy that you are going to fatigue faster than most people. Sure, some people do this and it doesn't affect their play at all... ask them how long they've been playing though, what kind of music they enjoy, etc.
The wasted effort will also limit how fast you can strum. It's pretty simple really. Your wrist is moving 4 or 5 times farther per strum than the average player to achieve the same results. This can be important if you ever plan on playing with other musicians.
Then there's the finger issue... One of the reasons people strum with their index finger is that it allows you to tuck the other 3 fingers safely against the palm so you don't double-strum, twang, catch a hangnail, etc., etc.. My solution is to hold my other fingers out rigidly to keep the out of the way.
Since you have 2 fingers and a thumb in use to play the same chords most people use one finger for; you are dramatically increasing the possibility of screwing up.
Basically, it just means that you're working harder than you need to and not really getting any immediate benefits from it. But who knows, it might all work out brilliantly in the long run. Lets face it, it all hinges on your playing style, the songs you play, and what you're willing to give up for any possible long term benefits. If you play music by the note it won't matter at all. Learning complex strumming patterns will be problematic though.