I agree about the nuisance of delays. There are some who don't prepare and some who do. I expect the fear of losing members, in a climate when clubs can't afford to lose even their crap performers, is part of the issue for organizers and MCs, etc. And, they can't risk being seen as harsh on people who probably aren't trying to inflict a probem on others: that's part of the democratic-ness that the folk-world values. Clearly, JC doesn't see the solution as "just b----y well tell them" (unless he DOES, but he's not gonna enact it!!)
As for instruments, while you can tune some to "close enough for folk" levels, you can't easily tune perfectly while others are performing; if you leave the playing-space to do so, you miss part of the previous song (and potentially create a disturbance by getting up and moving out) but others' songs might be what you've gone along to hear: that's part and parcel of a singaround.
I favour being upfront and telling the offender on the quiet: thay're not going to slap you round the face, nor raise the issue aloud with the rest of the room, in case the room disagrees with them!