No one uses more registers just to use more registers - in OoOe designs the main reason to use more registers is to reduce spilling and reloading. So in effect a compiler isn't going to use a register it has to save, unless in doing so it saves a spill+reload, which would result in the same number of load/store as without the additional register.

In-order designs have more reasons to use more registers, but again they aren't going to use more registers unless they gain something.