How many Pi's are we talking about?
1? For practice, OK.
10? Not worth the time or money.
1000? Maybe things get a little interesting, but its power efficiency isn't on par with a desktop CPU, much less a GPU. Not to mention it's ridiculously expensive.
I have two lying around so all two of them, I doubt I will make anything. Currently at work we have 16 computers with i5 processors and we're looking to potentially cluster them for crypto currency mining, any suggestions on what currency?