You bring up Volt Turn a bunch, but if I see a Magnemite I am immediately switching in a Ground type for that specific reason.
Also, once again, Magnemite will be switching in to absolutely nothing. Not rocks, not any attack at all, or Sturdy is gone, and it's subseptible. Chinchou, Magnemite's main competitor, will find itself switching in often, especially on Magnemite!
Also, again with Volt Turn, who's to say you wont get U/Volt Turned out on?
In terms of being outclassed by Chinchou, (because their roles are, in fact, very similar) the number of roles that Magnemite can effectively fill is two, while Chinchou can have up to 4. Magnemite effectively runs a Scarf set and a (pseudo)bulky attacker set. Chinchou can run both of these sets, plus a Cleric set and/or a RestTalk set. The best part about those two is that they can be combined, though that is not the most effective of Chinchou's sets.
Though Magnemite is still rather good, being outclassed by another A+ pokemon in typing, versatility, and hazard weakness should warrant a separation of the two.
EDIT: You know who else can switch in on Magnemite? Timburr. Simply take the standard spread, switch the nature to Careful, and then:
240+ SpA Magnemite Thunderbolt vs. 76 HP / 156+ SpD Eviolite Timburr: 10-13 (40 - 52%) -- 9% chance to 2HKO
(10, 10, 10, 10, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 13)
Magnemite A+ -> A