"...an articulated joint system has an advantage of being able to either step over or literally climb obstacles..."

It's also a pain in the can to maintain, especially under field conditions.

"...most heavy weapons carriers take time to re-aim, the gun turret or the vehicle itself needing to rotate to a new position. By using arm-mounted weapons, re-aiming within a 90? arc or greater could be significantly faster, just as a human can change the point of aim of a pistol more quickly than they can a rifle."

How so? A pistol can be aimed faster than a rifle simply because it weighs less. However, it's also notoriously less accurate. Then there's still the question of ammunition storage and feeding, especially with an articulated joint.

There are few 'general purpose' combat vehicles in the world today. There are plenty of non-combat vehicles in the military, but nobody wants a HMMWV or a C-130 when the action gets heavy. There are specialized variants of both of those for combat (and other) situations, but they're rarely anyone's first choice of weapons platform. Generalized systems, like human beings, don't do any one thing well and are at a disadvantage to those designed / evolved to operate in a specific environment.