There are some great blog posts out there (from yesterday and today) that are discussing the use and need for previews in theater. There's some great stuff from Rob and Kris, so go check it out.
Personally (and with SRT), I've always looked at previews more like an "invited dress". SRT has always had lower ticket cost (around 5 bucks), but since most of the audiences end up being friends and colleagues who comped in, we only make about 50 bucks at the most.
And although this has never happened, if something catastrophic were to happen, I would probably be inclined to refund money if need be.
The value that SRT gets out of it (I feel) is letting the actors get used to a non-tech-week audience. People who have NEVER seen a run of the show before, and the laughs or reactions are fresh and new - and sometimes in unexpected places (who knew they'd laugh at "x"???). This all assists the actors and the designers to help fall into the RYTHYM of the show. Pace can be the make or break part of a show, and getting a chance to test out the pace is a godsend.
Now, if someone saw an SRT preview, hated it, and then ran to their blog and posted about it - I would be SO DISSAPOINTED.... but I would have no reason to be angry about it.
By opening up for a preview, you roll your dice on the criticism.
Some folks, like the folks at GreyZelda, discuss why people use them at ALL.
Where do you fall in this discussion???