But if you want to explain to new players that "toggle abilities default as ingame except for weak armor because its shitty and people forget all the time to toggle off" then go for it.
I would actually be down for this :D
Really only zekrom would not understand
But in all seriousness, Weak Armor is more often than not detrimental to its users. 99% of the time, a Weak Armor user not named Mandibuzz is going to want to have it off. Honestly, my suggestion isn't too different from people wanting Sheer Force to be on by default: when SF was off by default, people got screwed because they forgot to toggle it on.
Weak Armor gets activated multiple times by multiple-hit moves.
96 fully-evolved Pokemon learn some multiple-hit move, and that's not even counting CAPs nor mons that get one from anime/manga/event. For those of you not great with numbers, that's a significant portion of the fully evolved Pokemon: 32.432% to be exact. (again, not counting CAPs or anime/manga/events) It's not too difficult to do Comet Punch followed up by some physical moves. Yeah, you'll be super fast, but that hardly matters when you're taking at least 4 extra damage from every attack. I've only ever seen Weak Armor toggled on on Mandibuzz, except for a couple of times when akela toggled it on his Onix and Omastar for teh lulz. Oh, and I think I saw Rediamond toggle it on once. As far as I can remember, from when I joined in Dec 2012 (more than 2 years ago!) I've hardly ever seen Weak Armor toggled on, or, after the change to toggle abilities, left on intentionally. Sheer Force should be on by default because that's what most people use it as. By that same logic, Weak Armor should be off by default because of its 9 fully evolved users (Garbodor, Omastar, Kabutops, Magcargo, Skarmory, Crustle, Vanilluxe, Mandibuzz, Aurumoth) only Mandibuzz prefers to have it on, as compared to the other 8, which only want Weak Armor on in the most specific of circumstances.