Thank you Jeri for expressing things so clearly

Shambles: You made comments about "singers" expecting "musicians" to be an "audience"
I don't play any instrument well enough to actively participate in "tune" sessions but I can enjoy listening to music. In a tune session I am therefore turned into "the audience" and like many others even some who are instument players but don't know that particular tune well enough, have no opportunity to participate at all. Some tune sessions have sufficient variety and interest for those who are effectively the audience to enjoy a whole evening of this. Many don't and either it ends up appearing that 4 or 5 people who know each other are using the pub to practice together and others are tolerated because it's a public venue, or it sounds like the same three (twidly didly) tunes are being played over and over and over again! This might be fun for those who are actually playing but as an "audience" it lacks something, which is why I rarely go to sessions that are intended to be primarily for instuments.
At singaround sessions, while you can get some (as Jeri said) who hog the session singing very long boring ballads many songs have choruses or refrains which enable most people in the room to join in if they chose to. In what I think of as a good session, there will a mix of individuals who's songs touch me and the buzz that comes from joining voices and sharing impromptu harmonies with others, to make a sound that seems to echo through you. This to me means that in a good singaround session you don't need to have an individual spot to have been able to join in, and not just be a passive, sit there and be entertained audience. To me instrument sessions tend to be more excluding than those for voice.
A good mixed session should not just be an instrument session that has the odd break "to (grudgingly) allow" the singers a go. A mixed session should be just that, a session that has a mix that includes: instruments playing without singers, singers singing unaccompanied, (comic) poetry and even (where invited and apt) accompanied songs. Unfortunately good mixed sessions are few and far between as they have a nasty habit of either becoming over formal or being hijacked for either song or instruments, which defeats the object of a mixed session.