I picked up a used Kindermann 6x6 medium format projector for about $100.00 off Ebay. They are quite common, inexpensive and use a tray system for 6x6 or 6x4.5 slides.
The lower cost of 35mm accessories allowed me to accumulate a large inventory of view screens, film cassettes, extension rings, filters, viewfinders, bellows, motor drives, and lenses that enabled me to configure my cameras to just about any assignment. The medium format or large format equivalent would have been cost prohibitive.
Swiftness of operation, size, flexibility, the list goes on and on...for me though it's the continuity one gets with a 36 exposure roll. I shoot 35mm and 120 (occasionally 220) with people exclusively because generally changing film holders in my 4x5 takes too long (anything longer than the 10 seconds it takes me to grab my alternate body or film back off the work table is too long).
Also the overall mechanical layout of 35mm cameras is something I enjoy. It's rare for me to find a medium format camera other than my Hasselblad or Rolleiflex that just "sits" in my hand. My Pentax 6x7 is alright, but not as comfortable as the others, and doesn't come close to holding a candle to my Nikons (F4s and F3s) or my Leica M2.
And needless to say, fast lenses, fast focusing, and fast shutters. Time between photographs is negligible, on a MF or LF camera there is a delay.
Holidays at my wife's family's homes everyone pulls out their point and shoots and fire away. When I used to pull out my Contax 139 with a 35 or 50mm lens and now my Nikon DSLR, all the women have a fit and don't want their photo taken. :D
The flip side: medium and large format images have vastly superior tonality. They just look better!