The only way I'm ok with a reduced magazine is if it's due to a limited grip size.
Example: I would not carry an 8 round magazine in my full size Glock 17. That gun is designed with a large grip for the sole purpose of carrying 17 rounds and that's how I'd carry it. Why deal with the wasted space?
If you want a semi with two smaller mags, carry a compact version like a G26. That's the only time I will voluntarily carry a smaller mag (unless camping in California has neutered my magazine
)