"CPU only served to poke settings into registers, do some trivial AI and collision detection, and poll the controller ports." <-- I know that's false for an absolute fact, demonstrably so in many cases. I know that the CPU doesn't do as much as some people think (especially on the SNES), but it's not as unused as you say.
I've seen some really heated arguments over SNES vs. Genesis, which seems ludicrous to me at this point, but it's definitely alive and well. Nintendo fanboys never seem to be able to get over the fact that the SNES was slower, and Genesis fanboys never seem to be able to get over the fact that the Genesis audio/RF quality/color palette was seriously lacking by contrast.
Saying both had slow CPUs is a gross oversimplification of its own. Of course they're slow by modern standards (hell, slow by 1995!), but at the time there was a pretty fair gap between them. Games that didn't use the enhancement add-ons were really prone to the infamous slowdown. And we're not talking retard basement-dweller companies that had games with slowdown, we're talking Konami, Capcom, etc.
The SNES's limitations were the entire reason these existed :
http://en.wikipedia.org/wiki/List_of_Super_NES_enhancement_chips
"As part of the overall plan for the Super Nintendo Entertainment System, rather than include an expensive CPU that would still become obsolete in a few years, the hardware designers made it easy to interface special coprocessor chips to the console. Rather than require a console upgrade, these enhancement chips were included inside the plug-in game cartridges. The presence of an enhancement chip is most often indicated by the presence of 16 additional pins on the cartridge card edge"
Pretty awesome idea really.
The real proof is in the games. Pier Solar is a great game in it's own right, but it's pretty ugly compared to even FF3, and we're talking about a homebrew title that was made using a 64 meg size never seen at the time on SNES or Genesis. And for whatever reason, unless the coders were REALLY good, most games on SNES were either slower paced, or suffered slowdown. There are exceptions to that, but some of the comparisons are kind of amazing.
Hell even ancient PC Engine did well :
https://www.youtube.com/watch?v=qOJk7sOcbMA
This game had loads of slowdown :
https://www.youtube.com/watch?v=0t6Mv_XML7o
so did this :
https://www.youtube.com/watch?v=KBRkqsX-YR8
The SNES Ghouls game was WAY prettier, but the Genesis one moved so much smoother, and it was far older.
I'm not saying that it makes or breaks the systems by any means, I love both of them to death. There are just some general rules that come across pretty clearly :
Genesis does shmups and super fast games better in most cases.
SNES has way better colors, and cool modes, and the games with additional processors are sometimes fairly amazing for what they are. And the audio, wow. Some used the Genesis abilities to decent effect, but it's nowhere near what was in the SNES.