It all depends on what works and were they are held.

The singaround/session we run in Hull at the Welly are run on a "dive in" basis.
This is working perfecly well because;

1.Most musicians and singers know each other
2.All give respect to one another.
3.Newcomers are given a hearty welcome
4.The more reluctant are given encouragemnet by the more outgoing of us.

I have run and been part of sessions that have been organised and held in not so quiet pubs, this has needed a guiding hand.

There have also been sessions that have been badly run were people do there bit and then think it is perfectly ok to talk and noodle through some one elses contribution.

And of course the ultimate were the organisor does his/ her bit then wanders to the bar and talks loudly to mates - as Leadfingers says "result chaos"

I have also seen organised sessions in noisy pubs were ultimately even the regulars have been won over to participate or at least listen.This of course does take time.

I believe the key to any successful session is mutual respect and courtesy for each other.