I know your question was directed to Tom, who has a much greater knowledge of developers, etc. than I ever will, but I think there are a number of reasons why one would choose to use coffee based developers.
1. experimentation
2. safe to use for people who might be sensitive to other developing chemicals
3. inexpensive
4. buy it at the local supermarket
5. easy to mix
6. fun
These are some of the reasons why I like to use Caffenol, others might have totally different reasons.

Your question basically boils down to this,"Why try something new and unknown when this works better?"
If we don't try, if we don't answer that question, how do we know for sure? Learning, trying, seeing is what makes photography worthwhile. At least for me.
It really comes down to choices that matter to each individual.