How would those SPUs be put to use at all next to a modern GPU? A GPU which can do the effects the SPU is used for now much faster than the SPUs can? I don't see the point.
Spare the GPU to let it do other things, perhaps?
It doesn't particularly matter always if the GPU could do the same work with greater efficiency or power efficiency - it can still be a win to do processing somewhere else if it let's you focus more GPU power on a bigger bottleneck elsewhere. I've seen it talked about more than once where devs chose to put certain algorithms on the SPUs even though they were slower than doing the same on a PPE or the GPU - just because it freed up those other resources to fry bigger fish that was bounding the overall performance, and so lowered the frame latency overall.
A lot of devs might not bother just because there may be so much power elsewhere that it's not necessary, but among devs who wanted to squeeze every last drop out, they'd find a use for them, I'm sure.
Not to mention those resources could also be applied in other ways - for example for OS tasks. Maybe the small SPU array could be used just by the in-game OS.
It would cost Sony some die area to include them, but they wouldn't go to waste at all I don't think, and if Sony was happy to go with a bigger die than their competitors, it wouldn't be the first time. It could be worth it...they'd be an awful lot more useful than the chips that have been needed for BC in past systems
(This is all assuming they wanted to go with a completely different/new architecture for the rest of the CPU, and SPU or SPU compliant design wasn't on the cards for it...)