As bad a reception that Sonic 4 got, I understand the numbering. It was the first fully 2D Sonic game on a console since Sonic 3 & Knuckles.
But the need for a Sonic Adventure 3? I don't get it at all. We've had numerous 3D Sonic titles since SA2 and we are well past that era. Would a Sonic Adventure 3 continue the plot? Nope! Heroes and Shadow did that. Would a Sonic Adventure 3 see the return of the Adventure gameplay style? Again, Sonic '06 and Unleashed had just about every element found in the first two Adventure games.
The only thing I can think of is the title itself, which is a pretty stupid thing to be clamoring for, or the return of the Chao Garden. Thing is, Chao were an extension of NiGHTS A-Life and were created as a way to take advantage of the Dreamcast's features. After the Dreamcast, there is really no point in having A-life returning to the series. Sonic Team went as far as they needed to in experimenting with A-life, and they really shouldn't be putting an emphasis on it now when the real focus should be perfecting Sonic gameplay.