Do we really need a Swiss Pass?

Hi
From the forum it seems every traveller is different so please assist with our specific travel plans.

We arrive in Frankfurt AM 28Sep11 and are considering a train connection straight from Frankfurt airport to Bern or a flight to Zurich then train to Bern.

We are booked into a hotel in Bern for 5 nts from 28Sep11 as a base to see Swiss Alps, Swiss cheese Gruyere, etc
Keen to do Golden Pass scenic journey, Interlaken, Jungfrau, Zermatt, Montreaux using rail system etc
Also noticed Giessbach Falls and Hotel Giessbach,Brienz looks interesting but will need to work out how to get to all destinations from Bern

Will also need to book a train from Bern to Paris on 3Oct11

We return into Basel 23Oct11 AM - leave for home out of Frankfurt late PM on 25Oct11 so have a few extra days for Basel Zurich Lucerne

So potential for lots of train travel / post bus / etc.

Swiss Pass looks to be what we need but what if we buy the pass then can't book a seat on the trips we want to do when we get there? Would probably do 1st class - 2 adults travelling together.

I've searched for some trips and found no seat available for reservation online in 1st class on selected days.
Would it be safer to just book the trips we want when we get there through the hotel? Then if the weather is poor we can choose a different destination on a given day.

Appreciate any advice you can give

By the way, a train transfer Frankfurt - Bern takes about 4 hours. I think that's more convenient than taking another flight plus a train Zurich - Bern.

OK but I'm still a little perplexed.
Example search on Rail Europe shows a wide range of time options for a trip from Bern to Interlaken in both first and second class on 1Oct11 to book the ticket on its own without a Pass of any kind.
But if I look for Pass Seats on the same date the only option is 6:04am.
Does that mean that to use the Pass on this leg the only option is the 6:04am train otherwise you pay the normal fare?

So it seems that one of these passes reduces flexibility and we'd probably end up buying individual fares anyway?

Thanks - I think it makes sense now and I can see some suggested day trips out of Bern on the RailAway Day Tours site that will help with our planning.
Thank you for the quick responses - back to planning!

You're welcome! Our site lists day trips and trails that might come in handy as well. Have fun preparing!