same reason as why I would level any class, it's fun to see my level increase and it could become more usefull in the future, now it seems that high desynthesis level increases rare drops maybe like 30%?
I genuinely don't know what "rare drops" are anymore, really, so, just curious about it. I'd consider things like the demimaterias, but, not even sure those are still used in crafting later stuff? Naturally, if it saves you some time/effort on some hard to get materials, that's great. I myself haven't been too worried about getting desynth much beyond ARR levels, since most items gained out of it beyond that just seemed like stuff I could get just as easily by other means.
this patch we have the material which you need for all new crafting/gathering gear, so you need to do less scrip farming to get them if you have levelled your desynthesis, prices are low thanks to casual desynthesis (about 40k each) but not so low that I would buy 200 of them with those prices.
If you want savage gear, play savage. -_-