While it certainly sounds like you know what you're talking about, it's pretty clear to anyone with a game-dev background you do not.
Cell's SPEs are actually PRIMARILY used as aids to graphics processing (T&L) by most developers. Look into how games like Heavenly Sword use the SPEs as part of its "faux" HDR or games like Killzone 2 use SPEs to implement deferred rendering for awesome smoke effects. The SPEs are, in PRACTICAL TERMS to PS3 game developers, very essential to the 3D rendering side of the console.
While RSX is "powerful enough" to do its own T&L, it cannot compare to the standalone power of the 360's Xenos chip. There are many reasons for this (6 fixed vertex shaders on RSX vs the unified shaders on the 360 which permit far higher vertex workloads, to the RSX's limited bandwidth vs the 360's eDRAM bandwidth, to triangle setup rates). On the PS3, developers need to leverage Cell in intelligent ways to draw comparable graphics to the 360. If an intelligent and determined PS3 developer really leverages Cell, it can make unparalleled graphic in the console world. The problem is, it costs a fortune in time and money to do it and very few developers can. It's simply not worth it to even attempt it for most developers.
As a sidenote, Cell is not at all good for most game AI for many reasons (not the least of which is the lack of branch predictors in the SPEs).
Additionally, people keep making the mistake of assuming the PPU in the Cell is basically the same as each core in the 360's CPU. That's not at all true. There are some significant differences, including native Direct3D format support in the 360's CPU to the new VMX128 vector units (which have 128 registers per context per core [6 total], vs 32 on the PPU) as well as additional instructions specifically tailored towards 3D games (like single-cycle dot-product instructions). The combined triple VMX-128 units on the 360 are still faster than most quad-core Core i7 in vector processing, so I'm perplexed by the notion that it's somewhat slow or underpowered from what I've read from some people.
If you're truly interested in how PS3 games use Cell, check out the Beyond3D community where PS3 developers post in detail about how they do what they do. And Cell is a major factor in 3D rendering on the PS3. It has to be.