Shattered Defense alone is the key here, I feel, given how it directly hampers Mortal Combo. Aside from causing massive problems alongside CS in how Arms is balanced and developed in my opinion.
Mortal Combo could be great, but as it stands to hit anywhere near decency you have to do as you do in PvP, by going CS MS WB MS for burst, but in reality in PvE popping WB this deliberately isn't good.
In for the Kill, in my eyes, suffers the Sweeping Strikes, Defensive Stance, Overpower, Rend syndrome: these are not good "talents" nor do they make for good ones.
Arms' talents are mostly made of abilties that were in the core kit in previous iterations of the spec, some of them with different tweaks.
As they were, those made for nice quality of life improvements. But as an optional choice they can't hold a candle to talents that outright improve the strength of the spec.
EDIT:
It all boils down to what the point of the thread is though: given how Rage income and spending is right now, either how Warriors generate it, the spenders or the talents has to be remade from the ground up.
Years of messing around and testing mechanics brought the spec in a place that's perplexing at the very best.