1. To have more options on what you can desynth
2. There are a few items that are only available via desynth

I have:
a. cln/fsh because I want to get the swim suits/waterproof cotton
b. gsm because it is easy to get ilvl70 accessories to desynth
c. alc because I've managed to pick up a bunch of primal books that I want to desynth for those materia

Once I get "enough" waterproof cotton I will probably change out cln for crp or bsm because I have a bunch of those primal weapons as well.