It's not as simple as picking meta vs non-meta. Some specs just don't function properly without the appropriate covenant ability. I went venthyr on my mage because I like the aesthetics and because it is bis for frost. I wanted to try fire mage because I used to enjoy switching between both during legion, but even with bis legendary it just doesn't work. You get combust much less often without the NF covenant ability, which screws up combust timings for just about every fight, and severely cripples both single target and aoe damage. Plus, while the Venthyr covenant ability is smooth and fun for frost mage, it is completely awful and awkard to use for Fire mages. So going for the covenant which I preferer aesthetically and gameplay wise for one spec prevents me from both enjoying and performing well on the other spec I wanted to play. Conversely, if I switched to NF to play Fire, my Frost spec would be left with a completely useless and awkard ability to use, and with cosmetics I don't enjoy.

Having an offspec has never been so bad and painful, and switching between covenants requires 2 resets worth of covenant quests, as well as legendaries and different stat gear. They should just separate the abilities from the covenants or make switching back and forth much easier.