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.
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Saving registers in functions is mostly irrelevant - you only save what you'd use, so saving more means fewer spills within the function.
Ah, but I'm sure that if you have more registers available, you'd use more registers. Up to a certain point. But what point? Just how many registers?
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.