M+ also drop flightstones (100+25 bonus), which also is the most consistent way of farming them. So no, m+ player do not need to farm rares.
But raiders do, as flightstones don't drop in the raid.
I know very few people that only raid and don't do keys. The majority of folks I know who raid also do keys. It's possible there's those out there that only do raids, but they'd find another source for sure (as evidenced there's more than one way to get flightstones).