I like the square 6x6cm format, so that draws me to use MF sometimes. (TLR user here)

I use 35mm if I NEED autofocus (such as photographing kids in existing light, etc..) and 1.4/1.8 lenses. I don't use it as much as other things because I'm kinda spoiled by the image quality of MF and LF. That said, the grittiness of 35mm is sometimes just what you want too.

I'll use the TLR for kids indoors IFF I am using the big studio flash where I can set the aperture and not have to focus constantly. I'll take the TLR for walks sometime with B&W film instead of the DSLR if I don't feel the color quality will do anything exciting in color digital. Lots of non-tripod applications for MF. TMY2 film really makes a high quality B&W negative in MF. I've made some nice 15" square prints on 16x20 paper. I don't decide purely on the basis of weight but my Yashica TLR seems lighter than my F4s+lens. A TLR is also deceptively cute and friendly if you are photographing people. Not the most versatile technical choice, but people love them.

Then for the next step up in image quality, I'll put a couple film holders (in antistatic bags) in my pocket and drag around my 4x5 speed graphic and maybe tiltall tripod. Stunning 16x20 B&W images. Every shot can be processed different if I want. Far surpasses the MF, the DSLR, etc... Not practical if you need to be quick. More DOF and lens options than the TLR. Can even do instant fujiroid pictures.