The problem is that it is hard to prevent it from being used as some kind of botnet/spamnet. There are still very few legit buyers of such CPU cycles.
This can be solved by sandboxing and giving only specific API to task developers. I think it can be done easily, because I already did this for python scripts and it worked quite well. For commercial usage maybe Java is better (high level language but good performance, possibility for GPU processing and sandboxing).
By the way there were some commercial projects like this. It always finished on low demand for this kind of processing.