As Teaboy has said, tyres make a big difference here.
We do have a 4motion, but do switch tyre sets for the winter - crazily VW delivered ours with pretty extreme summer tyres fitted (on factory 18s). Its easy to pick up a set of new M&S rated steel wheels from guys who've upgraded their vans to alloys at delivery..... at just the cost of the tyres (remembering the 180 needs the 17s). No real net cost, as both sets of tyres will last twice as long....
Of course, you'll have your summer tyres on in that wet muddy field at a festival...
Do we regret buying 4motion? No. Occasionally it would have been nice to have the option of the 5th seat (but putting it in and out and storing it would have been a faff - wouldn't want to camp with it); better fuel economy would been appreciated and there is no doubt the 4motion costs there. But no fear of getting stuck in a muddy field (we've also used the diff lock once - this is a another whole debate as this was standard in 2012 - now a £500 option on top of 4motion in itself)
All that said, if you know you're going to put your van in a situation where 4motion is going to be a benefit i'd say go for it. You'll see your money back on resale as they are much much rarer...
Simon