Here's my thoughts about Kangaskhan.
Opportunity Cost
There is an opportunity cost for using Kangaskhan. Adding Kangaskhan already makes your team susceptible to Intimidate and Will-O-Wisps, both of which are common elements in Doubles. Randy loved his Kangaskhan, and his teams tended to struggle with Intimidate, Wisps, and Ghost-types, since he liked to stack on other physical sweepers. He stated that he can usually play around things like Intimidate (and he usually does cuz he's gud xD), but it doesn't change the fact that a single Intimidate essentially removes Kangaskhan's Parental Bond boost.
Although most Intimidators (barring Mawile) don't enjoy switching into Kangaskhan's big STAB move, Kangaskhan cannot hit both opponents at the same time. Thus, mons like Landorus-T can swap the place of the partner that isn't being targeted by big mama and effectively nerf Kangaskhan. They can also easily switch into Fake Outs for easy attack nerf on Kangaskhan. With one Intimidate, Kangaskhan now fails to OHKO shit like Charizard and get roasted in the process, etc.
PuP exists to offset Intimidate boosts, but note that this is another opportunity cost of spending a turn doing piss-poor damage. Basically, Kangaskhan needs proper team support if it wants to pull this move off.
Splash Kangaskhan
I dont find myself including Kangaskhan onto many of my teams, because it does not offer any useful resistances, utilities (outside of Fake Out), and complementary offense (Normal-type offers 0 super-effective coverage). If I need a Fake Out mon, I just as easily add Hitmontop, which also offers Wide Guard, Intimidate, resistance to Rock Slide, etc. If I want a Pokemon that complements well with Kingdra, I will be selecting mons like Thundurus-T, which can wear down Water-types, Grass-types, Tyranitar, and Charizard with Thunder(bolt), HP Flying, and Focus Blast, while Kingdra wipes out Dragon- and Ground-types for Thundurus-T, not Mega Mom. The times I find myself adding Kangaskhan is when I am building certain offensive teams that appreciate Kangaskhan's Fake Out for a free turn of set up.
Sure you can slap Kangaskhan on a team with alright results, but the team may have benefited more from Charizard Y's powerful spread move that burns through Amoonguss and melts Steel-types; Mawile's Intimidate, resistances and Dragon immunity, and raw power; Manectric's Speed, Electric STAB, and Intimidate; Gyarados's Intimidate, resistances, and Dragon Dance, Water STAB, & Moldbreaker EQ; Garchomp or Heracross's raw wallbreaking capabilities and nifty resistances; Mega Venusaur's awesome tanking capabilities plus Sleep spam, etc. It's not as no-brainer as some of you make it out. Kangaskhan is NOT the unconditional best Mega Mon in Doubles.
Overbearing Mama
Mega Kangaskhan does not stifle my teambuilding process. I think this is the case, because it offers little utility outside of Fake Out, lacks resistances, and lacks a powerful spread move. I find myself paying more attention to Landorus-T (EQ, Intimidate, useful resistances), Togekiss and Amoonguss (Follow Me/Rage Powder and other support options such as Tailwind/Spore), Charizard Y (Heat Wave spam, solid special bulk), and hell Abomasnow (underappreciated threat that tanks most Water- and Electric-types while spamming powerful Blizzard; a terror in TR), before Mega Kangaskhan.
Unlike Bisharp I can cripple Kangaskhan with Intimidate, so I can deal with it like most non-Defiant physical sweepers. I am an offensive player, so I usually have no trouble beating down on Big Mama. The lack of Protect or Substitute on Kangaskhan gives me the security that my attack directed at Kanga's slot will go unblocked, which is a predictable factor that makes Kangaskhan manageable in Doubles for me. Bulky teams can make skillful use of attack / speed control options such as Intimidate, Wisp, Icy Wind, Thunder Wave, etc to cripple Kangaskhan.
The only thing that Kangaskhan has actively restricted in usage is probably frail Sucker Punch weak mons, such as Latios and Deoxys-A. There are plenty of solid options in Doubles to deal with Kangaskhan; I never felt I was making my team worse by adding "anti-mom" measures to my team, such as Fighting-, Ghost-, and Steel-types, all of which are great typing to include in a team with ample of options anyway.
What Kanga does best
I do admit that there are situations where Kangaskhan may perform exceedingly well. For one, Kangaskhan that foregoes Fake Out utility for its PuP sweeping capabilities pairs exceedingly well with Togekiss. Give Kangaskhan Protect or an extra coverage move over Fake Out, and it can rip holes with Togekiss's Follow Me and speed control support. The key here is to quickly eliminate/disable Togekiss, use your own Follow Me user (Jirachi!), and/or spam powerful Spread moves.
Another thing that Kangaskhan does well, is provide free set-up opportunities for certain threats. Many users have used Kangaskhan's Fake Out support to successfully set up Belly Drum with Azumarill or throw a Substitute with Kyurem-B, Heatran, or Jumpluff xP. This strategy isn't flawless, though. Quick Guard from Keldeo, Terrakion, or Talonflame undermines this. Having two Pokemon that can wipe Kangaskhan or Kangaskhan's partner also puts the Kangaskhan user in a bind, because she can only flinch one mon with Fake Out.
These two aspects alone may warrant suspect-testing Kangaskhan. However whether these cases will prove as evidence for Kangaskhan's banishment is dubious atm.