The point is whether it's adaptive, i.e. whether it has evolved because it is beneficial (such as our pain aversion), rather than arising as an unfortunate bug in the system (such as cystic fibrosis, a genetic disorder which seems pretty clearly not to be adaptive).
Of course PTSD is beneficial: it gets you discharged from military, for example, so the chances of you being in the situation where someone is shooting bazooka at you drop to zero.
In general PTSD helps you avoid traumatic experience in future.
In many interpersonal relationships, a trauma response causes you to become more liable to ending up in the same situation, not less.
Do not underestimate how many people end up reliving their trauma over and over because they can't overcome that response.
