Perhaps XT is a bad move as they move away from consensus too early. But I do not think it would harm Bitcoin. This will make core developers more aware, and reconsider if a solution is needed now or something needs to change. Perhaps it just needs to stay the same, but they will take a second look.
Worst case scenario would be that (1) the blocksize would become really a problem, (2) core developers did not solve it, and (3) there were no alternative solution.
(1) Is not likely to happen early, due to fees (as balu says)
(2) if (1) happens, is highly unlikely that core developers do not solve it
(3) Even if it happens, we have XT.
So I do not think the situation is a threat really.