Well, if you want to put defog on your bp team, then I suppose scizor is the best place to put it, but you really don't need defog. SR and spikes are nothing but a minor annoyance, and sticky web and tspikes are rare and blocked by sub (and tspikes are absorbed by scolipede). Besides, if your opponent is setting up hazards against you, they're probably making a severe error. Baton pass teams are by far the most vulnerable in the early stages, when they haven't got any defense boosts up yet. Allowing a free sub at the beginning is all that's needed to get the ball rolling.
There is a point to having multiple defense boosters, as you need to wall different things. It's imperative that you get defensive boosts up early. Assuming that there is a reason to use scizor at all, there is something which, out of the members of your team, only scizor can capably wall. If scizor is incapable of setting up boosts, then your baton pass chain is completely halted by that pokemon. It can do whatever it wants (running pokemon without boosting moves is the best way to get hazards set up on you in the first place!). Iron defense is completely non-optional. If you aren't using iron defense, you should be reconsidering your choice of scizor in the first place. If you really want defog, it should be either over sub or roost.