Thought I'd start this little debate as we approach E3.
Generally, mounts aren't something you're really dependant on in the starting levels. But as you grow, the need to cover long distances quickly becomes greater and eventually you'll end up sitting atop a steed of sorts. But how "soon" will that occur in ESO?
Making a strange theory before I make my prediction, I'll bring up that of PvP. Anyone can enter the Cyrodiil-conflict at level 10 (as it currently stands), and as you assault keeps and such, you'll probably find yourself wanting a quick transportation to the midst of battle. This will probably be the first indication of "dangit, I need a horsie". But at the same time you could argue that while a high level player might not have immense physical/magical superiority over a lower player (since everyone sort of scale to equal stats) - possession of a mount could be that superiority. While at the other end of that same stick, you can't have mounts so far away that only the last 10 levels has them. That would cause a level of imbalance that could mean that "in order to properly perform in a campaign, you'll need a mount". Which would render lowbies "completely useless" in PvP up to the point where they acquire mounts.
My prediction goes: They'll want your first experience in PvP to be about you and the other players - learning how to assault and defend - learning the ropes of the PvP oriented mechanics in Cyrodiil. And they'll want you to focus on that on foot for the first time. So while it aches me to say it (because I personally find it early still, but what the crowd will accept)... level 15-20.
What are your thoughts?