When you tune it up, it's crucial to tune the heads differently. reso higher, mostly). The square size somehow chokes the sound with two heads at the same (tighter) tension. The square size is flawed that way, but the rumble isn't the same with a 16x12 or similar size.
Jazz players have always made good use of the floor tom as a fill/solo voice, often it just sounds higher so you might not recognize it. Take Tony Williams on something like "Maiden Voyage". He also played rimshots on the floor toms fairly regularly. Later on, he used 14, 16, and 18 floor toms, in one kit.
Elvin Jones, Jeff Watts, Bill Stewart etc. all wouldn't sound the same without their (often multiple) floor toms.
Btw, deeper depth per diameter doesn't increase sustain duration, to the contrary. There is more volume to the fundamental tone, but the pay-off is shorter duration. You can't have more of both, as the energy from the impact is divided between both.