I can imagine that if the maps are large enough and the game mode is CnH or something along those lines where it is in fact more expedient to transport several players at once in a vehicle then vehicles could add another level of depth to the game. I certainly wouldn't want to see them on smaller maps but to simply write them off completely is a little short-sighted.
You really have to realize how large that map would have to be. Even Planetside2's
entire world map is only a few times the size of Blade Run. On a map that large, good map design would result in hi-speed trenches, not too different from Forgotten. Of course, given how everyone "loved" Forgotten, it would probably be even better design-wise to just make a smaller map. Now, if we pretend that Legions II were to have some component of MMOFPS akin to PS2, then vehicles as they are conventionally known might have a use. Consider though that the map would have to be many times the size of BR. The largest maps on Halo for instance in MP are probably just a few hundred meters in a given direction. PS2's world is tens of kilometers in a direction. The reason that gives a sensation of scale is in large part because of your movement speed (or lack thereof). You are an ordinary human in that game, more or less. In Legions you were presumably in the elite corps of a collapsing empire where you have the equipment that makes
you a vehicle.
Consider what vehicles have in other games: speed, unification (they can drop multiple people off at once), armor, and firepower. Legions already has those things and if you want unification, just go in groups. To improve upon the speed of Legions would mean that really any vehicle would have to be supersonic or it would be useless (but map design to allow players to hit sonic speeds would negate this). In regards firepower and corresponding armor, you carry a chaingun and a rocket launcher as your
default weapons. That already means that you personally have the power of a tank in any other game except that you also can fly at hundreds of meters per second.
Honestly IMO, there isn't anything that vehicles can do that cores or map design couldn't. I'm not saying I would oppose all vehicles ever, but I have yet to see a good reason to have them as they are conventionally known. To exist in Legions, they would have to be far different than what is seen in other games I would expect. It's not that I have any opposition to them directly, it's just that they are superfluous so far as I can tell and quite honestly, a lot of game developers add in vehicles not because the game is better for it but because players expect them. Look at T:A. Is it really better because of the Shrike, the grav cycle, and the Beowulf? I would say no.
Legions has a lot of core gameplay mechanics which IMO are spectacular and I have difficulty finding a niche for vehicles because Legionairres already occupy pretty much every niche. And I think this is part of the draw of the game. You aren't just infantry or a tank or a speeder bike or a helicopter or a fighter jet. You are all of those things in one at the same time.
And for the record, the views in this post reflect my own; I am not speaking for the team.