"But joining in is the essence of a session so this should be the exception rather than the rule."

Sessions vary. I know some where it's the other way round - every now and then it turns into a jam, either a tune, or everyone playing on a song, but most of the time it's people taking it in turns, singing unaccompanied or with an instrument.

Of course I'm all in favour of people tuning to instruments that can't be tuned, or to the standard electronic tuning standard, if that's the same thing, which much of the time it is. Or if a bunch of people are tuned to some other pitch, the newcomer tunes in to that. And if you aren't in tune with the rest, you don't play at the same time as them and they don't play at the samer time as you. Stands to reason.