The benefit is to save a few bytes, in a more general topic of code optimization that's beside the question.
On the ATxmega64A1, roughly 512 bytes (0,7%) are occupied by RETI statements. (124 bytes on an ATmega324, so it's more relevant for the more complex mcus.)
If you use many different interrupts then of course the interrupt vector table gets too fragmented to be used for program code, but if you are only using one or two then you are just wasting space with an "empty" vector table.
As this area is just like any other program code area (plus the bunch of disabled interrupts leading into it) and it is fully controlled by the compiler, it would be a fairly simple thing to add a compiler option similar to the $NOINIT option. I gather that no such possibility currently exists?
Whether it is worthwhile is part of the greater topic of code optimization. ;-) At this point I'm just trying to cover all (more or less) possible ways of getting the most out of your hardware.
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