It is pretty common for achievements to show how much people has it, and people become more involved if they can have something rare, there are many examples, the simples is Steam achievements. So, why not to have it in ESO?
From dev POV, it is new pretty simple database calculation, which can be performed every day/week, etc., so not costly. From analytics POV - you can compare rate with which achievements were gained before and after, then track average time spent in game on players who receives rare achievement and analyze if they started to play more while trying to get these achievements - so you can easily have analytical report on this change.
Who don't care - they can simply ignore it; nothing will change for them.
P.S. As far as I know, consoles have it, if it is true and you are console player, you can also vote based on your feelings on this matter
Your Friendly Neighborhood PvP Enjoyer (prior to U48)
Achievement Rarity 21 votes
Yes, I would love to hunt rare achievements!
Yes, it makes me a bit happier if I see I got rare one (e.x. on Steam), but I will not "hunt" them
Hate it!
1 vote