Mono Switch?

I have noticed that many hi-end preamps don't have a "mono switch". I remember my old receiver had not only a mono switch, but had left or right channel played through both speakers, and a speaker reverse [L to R; R to L...not a phase inversion switch]

So what's your opinion of the lack of or the inclusion of a mono switch? Extra circuity that degrades the fidelity, or a useful tool for mono recordings as well as a simple way to check for out of phase recordings?