Yes, this isn't like the MPC8641, where if you didn't use a Southbridge you didn't have certain peripherals.
The P5 is fairly complete without the Southbridge. It would bring us PCI slots for legacy hardware. It doesn't actually cost us any PCIe expandability (we actually we gain one X1).

And we gain several SATA controllers and sound output. You must admit that two SATA controllers is somewhat limited.

But it does add cost and complexity. You can already guess what side my opinion falls on, but the simpler option might be the more sensible one.