My thoughts on the whole button combo discussion:
Button combos in the whole of Halo 2's sandbox broke the balance between weapons. As a result they hurt the norm of Halo 2 gameplay.
HOWEVER, the MLG/competitive sandbox consisted of a very limited set of weapons... the BR being the most used. Because of this the button combos added a whole new layer of skill for the competitive side of things. Something unfair typically equates to little effort for tons of reward. The button combos in Halo 2 actually required a decent amount of effort for a decent amount of reward. Animation canceling used to be a glitch in fighter games and now they've become the norm because it adds a layer of depth to the gameplay that wasn't originally intended.
In Halo 2 the button combos were not intended, but they added that same depth. If Halo was built to incorporate the nuances of button combos with variables to control timing, damage drop off, cooldown, etc. they could add that depth to the competitive side without being too overpowering in the midst of the entire sandbox. The advantages of button combos could be controlled to ensure that they don't make particular tools ineffective. In essence button combos can be another tool in a person's toolbox. There would be times where double shot would not be preferred over a single shot... typically the added dexterity requirement lowers general accuracy. So at distance single shot would be best or for a one shot a single would be best. Double shots would be better at closer ranges and against players with more shields.
The added depth of button combos could be beneficial, but it also requires another layer of balance that needs to be looked at. And that is not something that is feasible with the additions of things like the Tact/Support packs. Ruling out button combos as a "broken" mechanic leads to missing out on the potential of added depth for the competitive players.
Gunz: The Duel is one of the more skill intensive games in existence and the metagame has evolved to animation canceling as an art form and the standard. K-Style involves comboing different actions together to cancel cooldowns to do things originally unintended by the game like wall climbing through wall jump canceling, double shotting with a shotgun through switch/reload canceling, dashing more often than intended through swing canceling, slashing more often then intended through block canceling, etc. The dozens of animation canceling tricks however are just added skills and have created a very devoted and very skilled competitive community.
Here is a video of Gunz finger movements... it starts with basic combos to insane combos later on:
http://www.youtube.com/watch?v=cQj33OEA_Y4
And this is a video of it in action in a 1v1. Keep in mind that players can only dash with their sword out and it normally has a slight cool down.
http://www.youtube.com/watch?v=euSoGDqprVQ
Just my thoughts. Button combos hurt the whole of Halo 2, but extended the competitive play. Intended integration can lead to avoiding the unbalancing of the sandbox, while simultaneously extending competitive play. However it is an added layer of balancing work and not really feasible with all the new stuff in Halo 4. Ruling out button combos in general is unfair as button combos can do loads to promote a more devoted and much more long term competitive community.