I find myself really wishing for proper multiple return values almost every
day, particularly when I work with maths heavy code, and especially for
efficiently returning error codes in functions I'd rather not throw from.
Many maths-y functions return some sort of pair; intersections return (ray,
t) or something of that type.
I'm finding HEAPS of SIMD functions want to return pairs (unpacks in
particular): int4 (low, hight) = unpack(someShort8);
Currently I have to duplicate everyting: int4 low =
unpackLow(someShort8); int4 high = unpackHigh(someShort8);
I'm getting really sick of that, it feels so... last millennium.

The point of 'proper' multiple return values is to return each value in
registers, in its own register type, using exactly the same register
assignment pattern as when passing args TO functions.
I don't think this causes any side effects to the ABI, since the arg
registers are already volatile across function calls in the first place.
It just means that the returned-to function can find its return
values already conveniently in an appropriate register, avoiding memory
access.

People argue I should return a tuple, but this isn't really the same, it
has hidden implications that complicate the optimisation potential. For
instance, tuples have an implicit structure/memory layout which can't be
ignored, whereas return values don't have memory allocated, ie, you can't
take the address of a return value without first assigning it to some local
syntactically.
The implementation of efficient tuple return values would be much more
complicated I would imagine too, and the rules are less clear; I can't
intuitively presume what behaviour returning a tuple of different things
should actually have in terms of register assignment. I also know precisely
how multiple return values should work, because it is exactly the same as
passing arguments to the function, but in reverse.

... just saying :)