If I were planning time in the Jungfrau Region - up in the hills that is - I would have alternative plans in case the not so unusual, unfortunately IME poor weather sets in - making hiking and higher altitude excursions a wash out.

So places like Bern - one of Europe's most cutest towns IMO and sadly overlooked by the tourist trampede to the Alps (the new Paul Klee Museum turns heads both inside and out) or even to Lucerne, which also can be enjoyed more in poor weather than a day up in the hills.

Which brings me back to the efficacy of a Swiss Pass over the Half-Fare Card - just for the changes in plans it provides - especially a consecutive-day pass - use it to hop to Bern or Lucerne if the weathe in the hills is poor - heck you could even hop a lake boat on Lake Thun as part of this excursion though the open-air seats would not be much fun.

and if going to museums in Bern or Lucerne then the Swiss Pass covers nearly all 100% - the pass IMO is much more utilitarian for the average traveler than the Half-Fare Card, which may well be better for someone just going to say Lauterbrunnen and not traveling but hiking - very few average tourists fit this profile.

We're sold on the Swiss Pass due to the flexibility it offers in changing plans ... in the event of bad weather in either Montreux or Lauterbrunnen we will head over to Bern for the day.

A good poor weather destination from Montreux IMO would be to Gruyeres - one of Europe's finest old walled cities, with a neat castle and a cheese making plant - and a nice train ride going via Lausanne and up thru vineyards from the lake. You can combine Lausanne too - the train station is right in the town center - trek uphill from it to the great cathedral and pedestrian shopping street - there is also the Olympic Museum, in this the IOC headquarters.