Getting Magnemite/Magneton in Power Plant is not much of a detour. It can be easily accessed by Flying to Rock Tunnel, then Surfing to the Power Plant.
Since Magnemite is pretty common, it wouldn't take much time, only about a few minutes.
Magnemite has some useful advantages over Zapdos. Both Magnemite and Zapdos do not learn Thunderbolt, and Magnemite does better without the Thunderbolt TM, having Spark as an alternative.
As I mentioned earlier, Magneton has superior typing, which allows it to wall a lot of stuff with its 11 resistances and immunities.
Lastly, Zapdos has the same catch rate as Articuno, a meager 3. Zapdos is going to require a lot of patience to catch, and you have to walk the whole length of the Power Plant to get to it, taking a lot of time, whilst Magnemite is considerably easier to catch and find, and does not require as big a detour.