How would it add strategy/tactics, by having a deactivated turret on the field, that can just be replaced by a new one anyway?
Because when you switch back it turns on. Think about it, if there is a good place for a turret but it's hard to get too once the enemies arrive.
Go there place it, come back change to something you need right now and once the enemies start attacking from that direction you can switch back and have it activate without needing to fight through enemies to place it.
It is something that an engineer can do anyway if they stay an engineer. This would just allow them (give them a choice) to be more flexible when the current situation needs a different class.
I'd like to see it be able to be destroyed when it's power downed though.
Yea if it couldn't be destroyed while powered down, then it would affect balance.