Really, the 1/2" plywood is structurally sound all by itself without any additional bracing. But it really does flex and bend a lot. Go to the hardware store and you'll see how easily it bends, then try and bend a sheet of 3/4"plywood. I don't know your playing style, but I beat the crap out of my cab when I play on it. When we get a good game of Street Fighter going that thing really takes a beating. I don't think you'd have to worry about it falling apart, just bouncing all over the room. Also, 3/4" gives you more "meat" for your screws (nails, glue, fasteners?!?) to hold on to.
Personally, Ive built 3 cabs - the 1st with 3/4" plywood, the last two with full 2x4 framing and 1/2" plywood for the "carcass". The cabs with the 2x4 framing and the 1/2" plywood are lighter than the 3/4" cabinet.