The no-OS is certainly no longer valid with the networking services etc. Still, even on the PS2 there are areas where you might or might not want to use the OS,

The no-OS is certainly no longer valid with the networking services etc. Still, even on the PS2 there are areas where you might or might not want to use the OS, or plenty of ways in which you can tailor your code more tightly to the machine. On the PS3 a perfect example would be use of the SPUs. The best engines have all sorts of functionality offloaded to the SPUs. Rendering, sound, joint processing, collision, particles, etc etc. Offloading each of these tasks is a fairly herculean amount of engineering as you must write elaborate SPU assembly modules (at least several programmer weeks for the very simplest ones) and the way in which the services interact with your main thread become fundamentally different. It would be hard to use these same architectural choices on the 360 and the PS3 as the 360 doesn't have SPUs (but has more regular threads).