I agree with most of your post the-bumper-car. I agree that HO needs momentum to win, and that we should be discussing strategy more than movesets in this thread.
However, I'm not sure "supporter" has an adequate role on HO. Something like Defog Latios or Rapid Spin Excadrill can certainly be used, but I wouldn't necessarily call that "support". I have never seen Whimsicott used to great affect on a HO team, but if you know of an example, please let me know. It's not that support pokemon are especially bad, it's more about the fact that you lose out on coverage.
Defog Latios and Rapid Spin/Stealth Rock Excadrill fall under "Offensive Support" to me, which HO really appreciates. Both also have notable resistances and immunities that make them easy to switch-in (HO is more willing to switch in their Support pokemon than their Wallbreakers/Sweepers). HO asks a lot of its Support pokemon, and one thing I've noticed is that almost all of them need to have decent-to-great attacking stats, making Taunt a nearly useless move against the whole playstyle.
For example, let's say a HO player starts the team and sacs his Deo-D for hazards. Afterwards, he sacs his Whimsicott with Memento. While he may be able to set up afterwards. The HO player is fighting a 4 vs 6 battle. If that sweeper is walled by even one pokemon on the opposing team, the HO player will probably lose. In my opinion, you need all the coverage and power/speed you can get on HO, so all five members need to all out wallbreakers, sweepers, or revenge killers (or fulfilling multiple roles).
Well, first off I don't think Deo-D and Whimsicott should be on the same team? As you mentioned that's way too many non-full attackers on the same team. Whimsicott makes a great counter for opposing set-up sweepers and as a way to safely switch in a sweeper who doesn't have that many good resistances/immunities - his Grass/Fairy typing actually gives Whimisicott many decent switch-in chances against many notable threats in the meta, especially since HO lacks an auto-include Fairy-type to run on their team (their choices are Azumarill, Whimsicott, M-Gardevoir, and Clefable - they each fill different roles, with different opportunity costs).
On my HO team I use Banded Talonflame as a wallbreaker, revenge killer, and late game cleaner (sweeper). He fulfills all three roles extremely well, which makes him a versatile pokemon. I also use Aegislash as a spinblocker, wallbreaker, and revenge killer with Shadow Sneak. Once again, he fulfills three roles. Coverage is the number one concern on HO, and I think the best HO teams have excellent neutral/super effective coverage while keeping momentum.
Another big change I noticed on GenVI HO is that thanks to the rise of priority in the game, HO needs revenge killers who can shrug off the priority and still kill. This is why Talonflame and Aegislash are both so good, and why Sucker Punch is still valuable.
Lastly, I think sacking pokemon can be effective on HO, but switching is a necessity as well. Primarily when you assume your opponent will switch to a pokemon that walls yours. Then a double switch is necessary. It's ironic, I love HO, but man is it stressful to use at times. If you mess up in the beginning with your Deoxys or lose one pokemon to prediction - you're probably going to lose a majority of the time. True HO relies on perfect play from the player, but makes me question as to why it's such a viable style this gen. Can anyone explain why it's so good when it seems to high risk/high reward? (For example, in fighting games or other strategy games, high-risk/high-reward is usually frowned upon and crushed in the competitive community.)
In my opinion double-switches are different from vanilla switches, and part of HO's gameplan is to convert switches into double switches. HO also is really dependent on having a good lead/early game, and I've found that I've struggled to win games with HO teams where I was at too much of a disadvantage or made too many games at the start.
HO is good because unlike other high risk/high reward strategies in other games (or even in Pokemon), it's not that match-up dependent. It still needs good plays (and prediction) to win, and the checks/counters for HO are useful against other match-ups as well. There's also the fact that HO, unlike other high-risk/reward strategies, isn't too luck dependent, and once they've locked up the game they are pretty much guaranteed to win, unlike, say, SwagPlay, where after a few wrong coin flips your opponent can end up sweeping your team thanks to YOUR boosts.
