If I were playing with someone who was out of tune, I would not be comfortable playing with them but I would also not be comfortable in informing them. I would not see it as my place to do so.

If one did think that it was your place to do so, it could be done tactfully and privately, not loudly shouted across the room. This was the case in the instance I referred to.

Tuners have indeed made it easier to tune up together. However on non-fretted instruments like the fiddle, a perfectly tuned (if there could ever be such a thing) fiddle can be played so that it sounds wildly out of tune. Certainly the case when I attempt to play fiddle.

In truth when the whole session is playing, the differnces in tuning are not really a problem and add to the (hopefully) harmonic soup.

Not the same I accept for smaller affairs and groups in performance, where it is vital that you are all in tune and feel you can both give and take criticism.

When performance is the pupose, the end is important. In the case of a session, there is no end result in sight, as it is the process itself that matters.