I was referring to the performance gain from the use of the second thread. I haven't developed for PS3 so I don't know hard numbers, but on modern x86 2 threads on 1 core only gives you ~15% performance boost over 1 thread on one core. The incentive for using that second thread is lower than for a true second core.
You're talking about the OoO Core family, which is extremely efficient at using its execution resources even with on thread, and thus doesn't gain much with SMT. In-order architectures are a different story. Atom gains around
50% on Cinebench R10 with SMT enabled. PPE will be more efficient as it will stall less often on its LSU, thanks to the big register file (32GP/FP/Vec (128Vec for Xbox360)).
Of course, on the CELL you might have ignored the second thread if you went wild on the SPUs, as the additional performance of the PPEs 2nd thread would be negligible compared to 6 SPUs. But for the Xbox360 I don't see how programmers will generally have ignored at least ~40% of available CPU power.