Yeah, that's the thing, isn't it? You sink that much investment into a mainframe, you're going to be having better things to do with them than mine bitcoin.
Although, yet another idle thought; maybe IBM themselves have 'tested' a machine, if anyone.
If you need to to lots of relatively simple things very quickly but also very reliably. You'd not use a supercomputer to process credit card transactions for example, you'd want a mainframe for that. And you wouldn't use a mainframe for weather forecasting.
Why on earth would they do that, the depreciation on the machine would be higher than the amount of $ generated that way and that's ignoring the cost of the energy and the facility to host it.
Although, yet another idle thought; maybe IBM themselves have 'tested' a machine, if anyone.
EDIT: typos,grammar.