People clamoring for "Ulduar Hard Modes" would like to have a word with you. But anyways: not many players would want to die in the middle of a fight, but death happens regardless, eventually. Look, the point is: if the tinker, outside its mech, can still tank "effectively" or "just as effectively", then that makes the mechanic
pointless. But if the tinker becomes greatly diminished, like a tank druid shifting back to humanoid form, then the mechanic (and the tinker) becomes a
liability.
No 'ifs' or 'buts' about it.