You would still have to prepare for those megas because you don't know who will be using them, even if there is a mega stone limit on teams. Mega pokemon are recognised even by Game Freak as something extraordinally strong, and that's why they limited them to 1/6 pokes per battle. And that requires you to pick a mega with care, and try to not lose it too easily in a 6v6 battle. In 1v1 however that limit doesn't matter because you can only use 1 poke anyway, which is why I think that in 1v1 mega pokemon that are too strong compared to the rest of the pokes should be banned, even if those same mega's are allowed in OU. Some of them that come to mind are the mega charizards, gardevoir, mawile, and salamence.
Regarding mega salamece, something I'd like to add to what
LaxLapras said is that a mixed set easily destroys dedicated walls, while dedicated physical walls already have a hard time dealing with a purely physical megamence in the first place. A mixed mega salamence set is simply not manageable from a defensive point of view. And the fact that it's so similar to rayquaza only proves how much it needs a
ban.
1v1 right now has become like a less diverse ubers, a bunch of pokemon much stronger than the rest are allowed, and they are used to counter each other, which is where I think the idea of unbanning blaziken came from, to add diversity and mix it up, and its already not even that much stronger than the common pokes used, it fits right into this little 1v1 ubers that's going on right now, so might as well unban it right? Mega mawile, kyurem-black, the mega charizards, mega salamence, mega gardevoir, and greninja are all contributors to this "ubers state" I'm talking about, and I suggest being open to banning them so that a lot more pokes can become much more viable than how it is right now.
And more specifically regarding unbanning blaziken, you need to look at what it does to pokes beyond of the most commonly used pokes that were mentioned here to justify its unban. It has very good coverage with moves like thunder punch and stone edge, it could be either special or physical, it could be choiced or have life orb and swords dance, it's really unmanageable for defensive pokes. There's a reason uber pokes are in the uber tier and I think we should discuss all the relevant uber pokes allowed in 1v1 right now and question their right to stay rather than unbanning more ubers. What about 1v1 makes them acceptable here and not in OU?
1v1 started out in XY with all ubers banned, why weren't new ubers banned? Something banned in OU is a strong indication of it being too strong for the rest of the pokemon to handle, unless the reason was something very irrelevant to 1v1, such as mega gengar's shadow tag.
I'm guessing it's because the policy is that people should post in this thread for something to get banned? But since most 1v1 players don't bother posting here, I suggest that if something
not irrelevant to 1v1 gets banned from OU it gets banned 1v1, and it should take people to post here to get it
unbanned.