My number one consideration in bag building is the ease with which I can obtain/replace the discs in the plastic, weight, and color(s) I prefer. For that reason, I only go with stock discs, and I always buy multiples in exact weight/color/plastic/production run if at all possible. I usually buy a stack of 10+ at a time.
That said, my second consideration is having as few molds as possible and still cover my shots.
My bag has been:
2 dx aviar p&a
2 x comet
2 champ teebirds
2 champ leos
1 kc roc
1 champ firebird
6 molds, but I really only use the roc & firebird as utility-type discs...they've earned their spots time and again, but I'm really only using them maybe once or twice a round on most days. So, really 4 molds cover most of my shots.
I carry duplicates in different stages of wear (one beat to hell, the other fresh) and that helps me obtain a wider range of shots. I used to cycle discs a bit more but I eventually realized I lose discs too often for that to be super effective so I ended up with the "one fresh/one beat" compromise.
Personally, I think a simple bag can force you to learn shot-shaping, not rely too heavily on niche discs (i.e., discs that only really serve one purpose in your bag), and help less-skilled players remain more consistent over time (because they've got a better idea of what each of their discs is going to do on accounts of the increased familiarity.
Really, it's not about being "faddish" or anything like that. It's just a preference that positively affects my game all the way from easing the hurt on my wallet (not "having to" try out all the new molds is a godsend) to helping me remain more consistent in my shot selection and execution.