I'd argue a necessity of either Synthesis or Giga Drain. The latter has the obvious advantage of giving a STAB with existing PP, while the former has the advantage of more reliable recovery.
I agree, though I'd say Giga Drain, even more than just consistency (if lesser) recovery. It lets Serperior regain health while still attacking, which leaves fewer free turns for the opponent. Serperior obviously murders Stall, but its fast enough that I still want to keep its match with offense in mind, and Giga Drain offsets LO recoil without allowing an opponent to set up on Synthesis rather than be 2HKO'd by Giga Drain + Coverage move.
Why should Heatran fear switching out. Scarf Heatran is faster than Jolly Dugtrio if I'm not mistaken and dugtrio does NOT want to be burned. Then again, Specs Heatran's Fireblast kills Dugtrio outright if it switches in. I guess my point is, Magnezone has a much easier time switching in.
I think it's still a matter of team building. Also, Scarf Heatran is faster, but Dugtrio is still usually hitting it for 4x
252 Atk burned Dugtrio Earthquake vs. 0 HP / 0 Def Heatran: 224-266 (69.3 - 82.3%) -- guaranteed 2HKO
Dugtrio does have Sucker Punch among its options
252 Atk burned Dugtrio Sucker Punch vs. 0 HP / 0 Def Heatran: 30-36 (9.2 - 11.1%) -- possible 9HKO
Which just barely eeks out a KO with Rock damage
With Rocks, Heatran can't take a hit from LO Serperior anymore, even with its 4x resist, and is within easy kill range for priority. If Heatran goes out, Serperior's free to clean, since that last bit of Heatran's health comes off if it switches into it again.
252 SpA Life Orb Serperior Leaf Storm vs. 0 HP / 4 SpD Heatran: 45-53 (13.9 - 16.4%) -- possible 7HKO
And this is all the Scarf Set. Any other set is outsped and KO'd outright by Dugtrio.
It's a matter of team building, but I find my teams have bigger problems with Heatran than Ferro or Skarm. Also, it should be noted that dealing with the others requires not just Magnezone, but something to Knock off their Shed Shells, something the opponent WILL be aware of and be cautious about keeping in on potential carriers.
Also, I think you're missing the point with Dugtrio. Magnezone has an easier time switching in and staying alive against trapped targets, but Dugtrio doesn't need to do so. Dugtrio's one job is "trap Heatran, kill Heatran", and any other inflicted damage is usually a bonus. It doesn't enjoy being burned, but Heatran isn't going to burn it without being crippled to the point of losing as well (Scarf Tran runs Fire Blast, no Lava Plume or WoW, so no guarantees even). HP Ground deals with Heatran, Magnezone while HP Fire is better for Ferro, Scizor, Pinsir (in the absence of HP Rock), Celebi (Hit harder than DP, and stops Nasty Pass shenanigans), Skarmory, Breloom, Chesnaught, Grass types in general.
Granted, HP Ground's targets are decently common, but the things HP Fire helps with either tend to be harder stops, or are better able to exploit free turns while walling (Laying Hazards, Setting up, Defogging, or Subbing for one of Breloom's sets). Heatran and Magnezone are also relatively easier to prepare for: since they share similar weaknesses and both are slow, you're unlikely to see them both on the same team, which means Dugtrio only has to deal with one per team as Serp's partner.