Ya know? That's a hard call. I find the .25x3.25 sheet films a PitB to process, handle, etc. But I still do it occasionally. Someday I might find a reason to use it and I will still want it there. I prefer processing and printing 120 format but the 2x3 sheet film is still in my camera, and refrigerator. I would like to use my 6.5x9 Trona some day and the only way that will happen is with a spring back for 2x3 sheet film holders.
6.5x9cm is very easy to process, in JOBO 2509 reels.
While we're on this subject - does anyone know of a Polaroid back to fit a 6.5x9 camera? I have a Voigtlander Avus.
I was on a similar quest for a polaroid back for my horseman VH, which is 6x9cm, and I eventually concluded that putting on a 4x5 enlarging back with ground glass is a very good way to go. Then you can use any old 4x5 polaroid holder.
It would also be possible to adapt a 3.25" polaroid back from something like an rb67 for your purposes; that would involve a little engineering. But at least those backs are inexpensive.