Ha ha, even without 360, they are forced to use Cell if they want to do something ambitious/advanced. Teh Cell is designed to be the star of the show.
From what I hear, the first few Insomniac games relied mostly on RSX for graphics work (No fancy SPU post processing).
I guess that depends on what you consider ambitious/advanced , there are some who've argued that Cell may have been able to do some ambitious/advanced non-graphical tasks if it wasn't helping RSX keep pace, but I digress.
Regardless, the only reason why I mentioned platform parity was due to assurdum's claim. We all know Cell is PS3's strength.