To Clarify, the Cell's SPUs can act as very powerful and flexible stream processors. This due to very high internal bandwidth and very high speed dedicated memory available to each SPU.
I think the point people were arguing over MikeB is that current top GPU's on the market are built with stream processing as a primary function and hence are leagues ahead for the task. This isn't a slight against the Cell in the PS3 though as it has to be versatile in order to be flexible as you say. Graphics wise it would be more efficient to not use the Cell if you had the dedicated GPU hardware availble, as it often isn't on the PS3 the Cell is brilliant at improvising any shortfall while still having plenty of power to spare. The Cell can never fully take over the functions of a GPU until the number of SPU's are increased dramatically which unfortunately creates hideous bottlenecks elsewhere surely?
The Cell is only more powerful than top PC processors for certain tasks as you well know, there is a reason it hasn't made any progress in the desktop market. In bespoke hardware its a wonderful solution but the PC market is embedded with way too much legacy software for the Cell to ever become a viable alternative. The Cell is just a different type of CPU that is unbeatable when used for the right applications.