I switch films often and currently have around 25 varieties of B&W, both 35mm and 120, in my fridge.
My reasons for not being consistent vary:
1) I like to experiment; I'm not a professional photographer and I don't need to make deadlines (or any income) with my photography so I can afford to play with different film and developer combinations and I have fun doing that.
2) I can't pass up a bargain; Expired bulk rolls? Short ends of some weird copy film? A brick of film somebody is clearing out of their freezer? I'll take it! I've shot thousands of trouble-free frames with expired film and it saves me a lot of money, allowing me to shoot more. (I will NOT, however, pay the absurd higher-than-new prices some people are charging for expired film. That's crazy.)
3) Different purposes; like just about anyone, I'll use a film that's appropriate for the task. I'm not likely to use 1600 at the beach or 50 in low available light.
Among the huge variety I still have favorites which I guess I can 'justify':
Kodak (old) Tri-X, Double-X and Plus-X - The tonality of these films, especially in the mid tones, is just wonderful. I know Plus-X is gone and Tri-X has been reformulated... again, but Plus-X is what I learned on and that's been ingrained in my consciousness as what B&W is supposed to look like. I've learned to like other looks over time but Plus-X and old Tri-X still feels like home to me. Now I find I'm getting a similar thing from Double-X (Eastman 5222). I have a stockpile of Plus-X that will eventually be gone forever and I've learned to like the new version of Tri-X. Thankfully, Double-X behaves, more or less, like it always has. I usually develop these in D-76 1:1.
Orwo UN54+ and N74+ - These two films approximate the tonality of old Plus-X and Tri-X and the price for bulk is very attractive. The grain isn't the same as the Kodak films they mimic, but the overall tonality is right in the ballpark. These get D-76 1:1 or HC-110 dil E, D or H
Rollei Retro 80s - Absolutely the opposite of the Kodak tonality I was just raving about. This film (really Agfa Aviphot Pan 80s and really much slower than ISO 80) has incredibly fine grain and has much less going on in the mid tones than Kodak. I associate it with a more modern look and I really like it when I want great detail. I develop it in Rodinal 1+50.
Foma 100 - I like this film. It's cheap and it has a look that, to me, is somewhere between the squashed mids of Retro 80s and the older look of Plus-X. I haven't used it in a while (I've been shooting Orwo UN54+ instead) but I was very pleased with it in both Rodinal and D-76 1:1. In Rodinal it reminds me of old Agfa stocks, which I suspect is what it was based on. It's an excellent all-purpose medium speed film.
Kodak HIE High Speed Infrared - I have a stockpile of this stuff that I'm parsing out slowly. When it's gone, it's gone. I absolutely love this film and no IR film before or since has come even close to the same look but I doubt we'll ever see anything like this made again. I'm going to have to come to terms with the Rollei IR stock soon.
Ilford FP4+ and HP5+ - I didn't like these films as much as their Kodak counterparts back when I was getting started 25 years ago but I've come to appreciate them now. I see them as being much more even across the board - they have less punch in the mids - and they're really nice for portraits. Now, if only I were a better portrait photographer....
There are plenty of other films I enjoy using but these are the go-to films that I reach for again and again. I realize that's 10 films, but what can I say? I've got a short attention span, I guess.