I agree, there's a lot of competition for a Mega-slot, and when you look at a mon like Raikou who can do a lot of what M-Manectric does without the mega slot, it makes it difficult to warrant.
So I'll quickly outline what would make it worth using over Raikou.
- Enough speed to revenge/force out Talonflame, Greninja and Scarftar (you can only Intimidate+volt switch Scarftar though)
- Powerful fire coverage a la Overheat, Flamethrower etc.
- Intimidate, which is the main reason. It has so many uses, forcing mons out, stopping sweeps, etc. You can abuse it pretty safely with volt switch or even just hard switching.
It doesn't seem like much, but it really does have a lot of utility in practice. It just does a lot more work in that one teamslot. I understand that Raikou has it's own things it can do, like CM, but that's a different role and can't really be compared to M-Manectric's job.
EDIT: Response to you Celtic
Now, Mega Manectric is only truly walled by bulky Ground types due to the nature of Volt Switch. It can always cause chip damage while switching when faced against something that has decent special bulk, and that has two side effects. One, 3HKOs become 2HKOs after enough (sometimes as little as one or two) hits, and two, you can gain a favorable match-up, giving you the upper hand with regards to momentum. Mega Man does need a decent amount of support in the form of hazard control, and you also need to think about what will be coming in after you Volt Switch out. A strong Fighting type like Conk or Terrakion is almost a requirement due to TTar and Chansey being common switch-ins. A strong Water or Grass type like Ches, Azu or Keldeo is also almost required to deal with Hippo and a few other bulky Ground types that give it trouble. Now, it doesn't need support from specific Pokemon, but it does need support from certain types to function to it's full potential.
I disagree with some things you say here. There aren't really any grounds that can counter Manectric outside Spdef Hippowdon and Mamoswine above 70%, as the rest are easy 1-2HKO's due to 4x Ice weakness and slowness. Once Mamo is below 70% it dies to Overheat and deals a pittance with Ice shard in return.
I don't think you need a Fighting-type, one really good team-mate is Lando-I, since it's immune to Ground, and can demolish Chansey, Ttar and M-Venusaur. Manectric returns the favour by destroying Greninja and Talonflame who stop Lando-I from cleaning house. Actually Lando-T is another nifty partner who can form a double intimidate volt-turn core, and still handles Chansey and Ttar for M-Manectric.
Also idk what you mean about needing a decent amount of support, sure it wants to enter + exit battle a lot, but so does Greninja, Talonflame, Latis and it's no weaker to rocks then they are. And you don't care about burns or paralysis since you're an special-attacking electric type that volt switches 1st turn usually anyway, so you don't hang around to take damage. Manectric dosen't strictly need any support, but it's nice to have a ground immunity.