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I don't see this as a realistic or fantasy issue, it more along the lines of imbalanced.

This is a situation I just thought of:
Enemy A is being shot upon by Enemy B. Enemy A notices that there is an unmanned tank sitting 10 meters away so he runs over to it and has 100x more armor and 100x more fire power instantly. If there was some delay to the entering or start up of the vehicle, Enemy B could have a chance to escape or prepare himself for the encounter. Since there is no delay, enemy A blows enemy B away and walks/drives away.

Edit: More so an enter/exit issue, but same general idea applies to be equipped for any situation.