Plywood is more stable "in a sense" than typically dimensional lumber and rough sawn lumber, but it's really not stabilized in the same way that a knife handle material would be stabilized. It may not necessarily warp or crack on you, but it will retain moisture a little more readily. Even ACQ or "pressure treated" lumber will take on some moisture, not to mention a potential chemical reaction with the blade steel.
Another thing to consider: most construction grade plywood is full of voids and fillers, and you may have to search before you get two consistent pieces to make scales out of.
As far as I'm concerned, a standard pair of black micarta scales is fairly cheap, super stable, and finishes out great. Well worth the 4 or 5 dollar investment per knife if you're gonna make anything you want to last and be proud of.
That being said, I suppose you could practice with it, if you were so inclined. You could probably even stain and seal it to make it a little more durable and moisture resisant, but by that time, you've probably spent more time, effort and even money than you would have on a set of micarta scales.