AMR's default stat weights and suggested information is notoriously incorrect. You know there is an Arms guide posted here by the way...
Mastery is supposed to be a good stat for Arms, however, it does dick all for AoE and doesn't affect any of our AoE abilities one bit. I've argued at length about this but the given response is "that is why you have two gear sets" which I think is asinine, especially considering the whole accessibility, and being able to use items when you get them fad the game is going through, but it is what it is; I would not expect it to change anytime soon.
Because Mastery is such a bad stat for AoE, you have two choices when deciding to play an Arms Warrior:
1) You are going to play Arms 100% of the time.
In this case you should either:
1A) Build two gear sets, one with Mastery for Single Target/2 Target Cleave and one with as little Mastery as possible for AoE fights
1B) Decide you dont want to go through that trouble and simply avoid Mastery, so that you can approach every encounter the same without changing gear (my suggestion for most players).
2) You are only going to play Arms for sustained AoE fights, as that is what the spec is best for.
In that case you should:
2A) Simply avoid Mastery.
Luckily we don't need to care about Mastery much anymore, thanks to the low amount of stats we have and the low rage generation that comes with it, Crit poops all over every other stat, including Mastery. After Crit, the rest of your stats largely don't matter and you'll generally take whatever is available.
The general priority is STR > Crit > Mult >/= Mastery > Vers > Haste
Haste scale factor is generally poor, however there are a couple Haste breakpoints to be mindful of. The first is fairly easily reached without gearing for it, and the second sacrifices too many other good stats to be worth chasing at the moment. The breakpoints will generally occur naturally and should only be considered when you are only a handful of stats away, when changing a single enchant or piece of gear will get you there. They are 10% and 25% Haste respectively (2 MS during CS and 5 GCD during CS).
You still won't want to lose Crit for Haste, but if you find yourself near a breakpoint and can trade a lesser stat (Vers), it could be worth going after it.
Edit: Nevermind about Haste, avoid it like the plague.
This profile is <1% (~90 rating) away from the 10% breakpoint and even Versatility poops on it now.
Regarding Execute
Once you hit 20%, the rotational downtime gets even worse, because your rage costs increase. You basically don't want to do anything except Refresh Rend, Colossus Smash, Execute when >40 rage, and use talents as appropriate.
This means that, time considerations aside, you will Execute when high on rage to keep from capping and wait for Colossus Smash to come off cooldown. Once Colossus Smash is close to coming off CD, you'll want to pool rage so that you can fit as many Executes inside the debuff as possible.
- - - Updated - - -
There are your stat weights for the T17P (pre-raid) Arms profile. Depending on gear, your mileage may vary but it should generally be similar.