There are 12.4M bitcoins, valued at something like $400 each. That means that the bitcoin network is tracking something like $5B in value. This is three orders of magnitude smaller than the value tracked by the computer systems of those major banks, even if all of their computing power were needed for the task. In reality, the vast proportion of that power is spent predicting the most effective enterprises in which to invest that value. Even accepting that high-frequency trading is of dubious economic benefit, there are still real economic decisions being supported by the computation.