Which is not a good thing. Only being a in a party because the party is forced to take your role due to some arbitrary mechanic or gimmick feels bad. And unless mitigation plays in some way into the economy of long-term output, where the two can be in some way exchanged and the carriers of one mechanic can at least still meaningfully interact with the others (however indirectly), that's exactly what tankbusters are going to feel like, an arbitrary gimmick -- a composition check. Being useful only because you are of X type, rather than any action you perform, feels bad.
:: For those entering the conversation mid-way, I am not saying that tanks are currently so underwhelming in their skill gap that they feel like you're merely place-holding for a gimmick capacity, but simply that the mere strength of a role, when one's efforts and skill have relatively little effect, will not be enough to make one's performance feel rewarding. It's not that bad, but I find it silly to think that it's impossible for playing a tank not to feel rewarding by nature of the necessity of a tank. It definitely can, as it has in other MMOs.
That would work if this were auto-chess. If your job were solely to enter the center of the room and sit there, invulnerably auto-turreting all enemies while you walk away from the computer, you would be valuable. If it performed other jobs in its output, it'd also be obligatory. But that does not mean that you, the one "playing" that autonomous would feel valuable.
But as long as we're expected to play the individual units in our composition, they need to feel impactful in their play as well, not just their presence. Only the choice of one among many pets, none of which would interact with their user's gameplay, can afford to care only about strength. AI can't get bored; players can.
