While I can't really say I hike with "newbies" alot, since I'm pretty much one myself (hiking about 5 years), I do hike very often with kids, making me the defacto trip leader. There have been times that we've had to abandon a hike, or at least alter it somewhat. During those times, that result was fairly self-evident and it wasn't so much a "decision" as a logical consensus by all.
(Example - such as deciding not to head over to Iroquois from Algonquin due to high winds or rough weather.)
Same thing in winter. I recall a hike, in which I was the newbie and we reached our pre-agreed turn around time and were not close (due to deep unconsolidated snow), we all just looked at each other, shrugged and turned around. No argument, or struggle with decisions at all. We reached the point at which we had already determined would "END" the hike, and that was that. That was a hike with fellow VFTTer's crazymama and JimB (and Jim's brother). We'd never met before the hike, but we'd set criterea beforehand and it never became an issue.
One thing, I believe strongly in, is that if you begin a hike together, you end it together (successful or not). The only deviation would be if you all agreed that that was okay PRE-HIKE. I don't believe in the "Go on without me, I'll catch up theory". The hike is more than the destination, and much of the joy of hiking with a group (whether family or practical strangers) is doing it together, IMHO.
That all said, I've (as we all probably have) hiked with people that have been struggling mightily during a hike, but I have NEVER felt I had to step in and make that decision to call it quits for them. It's not my place. Sure I've altered my speed and sometimes my goals at times, but have never deceided that someone else's hike was "over".
p.s. Coldfeet, you worry too much
Find a nice group and go have fun, that's what these mountains are about.