Good point. I recently bought 4 sheets of 1/4 underpayment ply at Home Depot because I couldn't believe the quality. I swear the good side veneer was as good as any birch ply I have ever seen and the back side was almost as good. And no warp, perfectly straight. I think it was about $7 for a 2' x 4'.

BTW: All the glues on plywood are waterproof. I was once talking to a rep and he told me, unlike the old days, it's not any saving in manufacturing to use anything but that.