There's just a little more to it than that too. The π cornerhorn is very well suited to the addition of the midhorn, both because of its layout and by the way the passive crossover binds each subsystem together as a whole. Everything fits nicely. In fact, the π cornerhorn was originally designed to be done this way.

You can also build a nice three-way with a baffle-mounted direct-radiating woofer. It will improve IMD, but in this case, I believe that the added complexity and cost isn't justified. I really like the four π speakers just the way they are.