Another point to Guest:
There's many a pub between me and the nearest Session. Not as many as once there were, but still well into two figures. Therefore, if on the odd evening you find you're not being regaled with the latest Gangsta shootout, you still have choices: it takes a broad palette to make a world. Whereas if you would hsut this down, you leave people with fewer choices: fitting an entire session into your home is likely to piss the neighbours off - unless they're invited!
That does not mean to say that in some areas it doesn't happen, or for that matter specialist groups don't - the Commun na Clarsaich does, for instance.
Another benefit is that a beginner can be developed in the traditional manner without problems - listening and learning to the tradition. Unlike classical music, where there is a warhorse approach to the major pieces (it sounds like this, and woe betide anyone who wants to change it - the result being the ochestral pros play it in their sleep, bored to tears at yet another "live CD" performance), folk music has seen a slow growth in style over the years, with a trad circle keeping the core form true, and developers innovating.