I read that the purpose of currency futures is "to fix an exchange rate
to avoid alterations in the future cash flows of the firm."

So it doesn't really seem like there's an absolute necessity for banks and other liquidity providers to make use of it. Therefore about what percentage would you say do make use of it?

You believe that currencies are fixed and therefore there is no money to be gained in trading it for value.

This is wrong, fx represents countries and there is a constant battle between countries who use Fx as a weapon to achieve fiscal/monetary polices and this is where opportunities lie to make a profit!