That's actually the way I prefer to do it, but on this particular project I didn't want to buy that much plywood and/or hardwood 2x2's due to not having a lot of cash at the time. Also figured the 3/4" void free plywood would be sturdy enough to keep me happy with the simple hardwood beam approach, and it seems to be.I have read that window bracing is the most effective and that is from a speaker designer, not a DIYer. The reasoning for that is when it attaches to the sidewall (in my case I dado'ed them in) they stiffen the flat surface much like an I beam is stronger than a plain steel plate. The dowel method only has one point on the panel so it does not stiffed the panel as much as a window brace.
I'll be paying a bit more attention to bracing on my tapped horn project though. Lots of leftover plywood around here right now, so I can afford to go nuts on it a bit.