In short: Not useful.
More elaborated: Messy, waste of memory-space and admin-effort.
Let me explain:
I've come across this discussion again and I considered myself lucky that Charles Hooper has done all the research already.
I'm summarizing his findings here:
Multiple Blocksizes do NOT offer Any Proven Advantage.
The theory about more efficient indexes and better managed cache is good, and I dont deny there is good reasoning behind it. But in practice, having multiple blocksizes and multiple db-nK-caches doesnt make a difference.
It is a waste of the extra (little bit of) work.
And most likely, you end up wasteing cache-space because you hard-divide the cache space into chunks and you prevent the Oracle LRU-mechanism from utilizing all available cache as it sees fit.
Of course, you may be the exception that proves the rule, and I would like to hear from you in that case, but until further notice, I'll stick with my motto:
Simple = Better.
