The reality is that in an MMO that allows you to zoom out to a Third Person perspective, using First Person is not very viable. In fact, in most MMOs, including the infamous World of Warcraft, you can actually zoom in to a First Person perspective. Is anyone in WoW using the First Person perspective? Of course not.
Third Person perspective provides a much more comprehensive viewpoint in an MMO. You can see mobs and Players comming from the sides and behind you and can avoid AoE attacks. Once a Third Person perspective is availiable, it means that the game (and mob-A.I.) will be balanced around players having that best viewpoint. It also means that anyone using the First Person "option" will be limiting themselves.
Ultimately, just like in other games, noone will use the First Person "option". The reason the First Person perspective is an option in Skyrim is because that is game that you actually have to AIM, so targeting does not just lock at the target like ESO. When you have to aim somewhere, First Person is equally viable and even superior to Third person. But thats not how ESO works.
Personally I would MUCH have rather have only a First Person viewpoint, which is far more immensive, than having the ability to shift to a much more strategic and comprehensive Third Person viewpoint.
In the official trailer, they present going First Person as a viable option based purely on preference. That kinda of makes me mad because developers and experienced gamers know better. First Person is not a real "option" when you have Third Person availiable in an MMO setting. Noone uses it in other MMOs and noone will use it in ESO.
It is just sad that it is advertized that way. Thats my 2 cents at least.