Using Mega Medicham without High Jump Kick is like visiting Detroit and not getting shot, or going on a cruise and then drowning, and yet people still don't run it (it doesn't even break 70% usage on the ladder.) Drain Punch can extend its longevity (it's one-shot by a large variety of moves anyway but w/e,) but not running HJK detracts from its main job, which is nuking the loving shit out of everything and lobbing off as much as 75% from offensive pokemon trying to switch in. Without HJK, you're also checked by Wisp X-zard and Quagsire walls you.
I like my medicham to live more than five turns in a battle, so I'll take sub+Dpunch over HJK, checking BalkZard.
As for quagsire though...
252 Atk Pure Power Mega Medicham Drain Punch vs. 252 HP / 252+ Def Quagsire: 163-193 (41.3 - 48.9%) -- guaranteed 3HKO after Leftovers recovery
Over which I'd prefer-
252 Atk Pure Power Mega Medicham High Jump Kick vs. 252 HP / 252+ Def Quagsire: 283-334 (71.8 - 84.7%) -- guaranteed 2HKO after Leftovers recovery
Which I understand and all, but-
252 Atk Pure Power Mega Medicham Zen Headbutt vs. 252 HP / 252+ Def Quagsire: 175-207 (44.4 - 52.5%) -- guaranteed 3HKO after Leftovers recovery
Assuming you're running a substitute set, like I do, you could possibly 2HKO Quagsire anyway. Loss of the more reliable 2HKO is a bit sad, but with the fact that quagsire has to waste a turn breaking sub while subjecting itself to a possible 2HKO kind of makes up, doesn't it?
More so, if it's going to spam recover instead of trying to break sub (which is more likely), there's the chance of Zen headbutt flinch. I agree, that's incredibly unreliable, but it's going to take a lot of zen headbutts if it's going to stay in... I'd even go on to say that the medi-user would be unlucky if he didn't eventually get a flinch.
Yes, it's very iffy, but I don't think it would be stupid for sub+drain punch to be a viable set.
Yeah Ghosts can switch in, but you liquify Gengar with Psychic STAB (btw, Zen Headbutt is also non-negotiable, you need it for Mega Venusar and Clefable) and Aegislash switching in to Drain Punch isn't all that great either, cause it's getting a free turn. It's better to try to Fire Punch it on the switch or just double switch.
Which brings me to a question I've been meaning to ask everyone.
Why do people run Psycho cut over Zen Headbutt? Is it the lack of contact?
Anti Rocky-Helmet Chomp/Ferro? (which are fairly rare anyway)
Anti KS aegi? (Which is impossible to see on offensive Aegislashes)
Or Anti Spiky-Shield Chesnaught?(lol)