I wouldn't argue about usefulness, but certainly Bitcoin mining is more directly economically rewarding than an indirect contribution towards protein folding progress for the field at large.

Every 1 hr of mining at X hash rate will yield a fairly predictable (though fluctuating) $Y of output, but every 1 hr of protein folding will not necessarily yield any direct or predictable income. From the perspective of profit-maximizing energy producers, I suspect they will prefer the former, unless someone can quantify or monetize the latter more easily and directly.

The amount of Bitcoin produced per hour is constant (or rather, designed to be constant between halvings). We don't need more miners.