My number are just a swag. However my point is, an instrument rating
in a light plane does a lot more to give you *options* flying VFR than
to give you the ability to fly real, hard-core IFR.

A lot of IFR flights people done in light planes, if they really look
back and think about it, can be done VFR. However a VFR pilot
shouldn't really attempt those flights because there're great chances
of running out of options. An instrument pilot can however fly in
really crummy VFR weather while still have options. When icing
condition, TS, or terrain forcing MEA to be be well over 10k like what
we have out west, flying VFR while keeping the options of getting a
clearance is often the safest way of doing it.

Not to mention the fuel and time savings in flying VFR in many cases.

I don't know if I quite agree with your 20% of instrument weather number or
40% of the VFR weather. I'd probably double those numbers so long as you stay
current. Of course... there are some that say that flying IMC in a single is unsafe
at any speed.