Here's what we know so far, the Paladin has an ability that is a counter-attack. That would make it slightly more ideal in a main tank role.
Since we don't know the other tanks reasons to have these subroles, it's really hard to say. However, considering Paladin still had supportive abilities, it could still make a fine offtank. It'll probably just lose some DPS for doing so, or need to engage in a more deliberate fashion to offtank.
Could be the case with the other tanks too...

I'd recall in Stormblood, Dark Knight was almost exclusively an offtank otherwise you'd end up spending your offensive resources on defenses, TBN was thrown onto the main and not yourself. Well this was often the case. Maybe something like that happens again. And it wasn't like Dark Knight couldn't maintank, and it had more identity.

If it causes more job identity, then it is probably worth it.