We were running alvarin exclusive before mounts made it in but switched to other builds a day or two before they did. And I think that's a large part of the reason you wouldn't see that. An alvarin only guild would be good when they are in places they can leverage cliffs, water, etc. but the moment they face cav on the open field every single one of them is dead.
I am very curious how naval combat will go. If water fighting ends up being a large component of it as we've seen in many other games with naval combat, alvarin would be deadly in water fights. Ohgmir will likely own the decks though as there is nowhere to kite on a deck and their slow/tanky melee build excels at fights like that. Plus they have the highest damage when exchanging arrows or spells. And a bonus to siege weapons.
Freaking Ohgmir and Alvarins may own the seas.