Since OS4 doesn't support SMP, the processor in the PS3 wouldn't be able to be utilized as designed and performance wouldn't be so great compared to Linux running on the same machine, if I understand how the PS3's CPU works.
The Cell BE processor in the PS3 only has one central processor. The SPEs are simplified specialized coprocessors that are designed to run tasks parallel to the the central processor. I'm not even sure that Linux (on the PS3) uses the SPEs.
As such, SMP support isn't a requirement on the PS3. I'm sure drivers could be written that would allow simple background tasks to be farmed out to the SPEs without requiring true multiprocessing support.
