While body-twisting may be an instinctually simple and efficient way to effect a turn, it doesn't make sense to augment this with a mecha body. The process of flaying mechanical arms around for this purpose is horrendously inefficient, not to mention that it takes those arms away from the actual work they are supposed to be doing.
In a mobile suit, the "actual work they're supposed to be doing" is really just holding onto, aiming and shooting artillery-sized weapons at warships and other mobile suits. Really, using the arms and legs for maneuvering takes away from the mobile suit about as much as it does in an actual human doing the same thing; a mobile suit really only needs to have one arm free, and even then only when it is actually in the act of shooting at someone (or, alternatively, deflecting incoming fire with a large handheld shield).
Although one does wonder why there should exist the need to pivot rapidly.
Same as the need for fighter aircraft to
turn rapidly, especially in extremely close combat, which is exactly what mobile suits were intended for. As a little background, the entire premise of mobile suits in the first place had to do with Minovsky particles (or N-jammers if you're in SEED) that basically neutralized radio waves except at very short ranges. Fighters and warships that depended on BVR weaponry would suddenly find their radars graying out, and next thing they know they're surrounded by heavily armed mobile suits that can turn on a dime without having to use thrusters or cancel acceleration. Plus, there's the fact that a mobile suit that runs out of ammunition can pull out a giant battleaxe and literally hack an enemy warship to pieces.
On the other hand,
Why not have full 4pi coverage of the surrounding battlespace, all at once?
The Zeons tried that with Mobile Armors. These were basically one- or two-manned battleships using various advanced control systems, including telepathic interfaces and powerful AIs. The most exlpicit examples would be the Nue Ziel and Big Zam, both of which were a) fucking huge and b) bristling with energy weapons, projectile weapons and--in the case of Nue Ziel--close-quarter melee weapons so the mobile armor could double as a kind of battleship-class wrecking ball.
The problem with mobile armors turned out to be their lack of agility. No matter how many thrusters you pack onto the thing, it will never has as much stamina or quickness as a mobile suit, and is--like a warship--vulnerable to close range attack.
In other words, if mobile suit is a P51 Mustang, mobile armors are B-17s. The heavy bomber may be bristling with machineguns and bombs (and, in a more modern battlefield, air to air missiles) but its lack of maneuverability means it will still be cut to pieces when smaller fighters get close enough.
Really, the most valid-sounding reason for a mecha in space would be if a transition needed to be made from free floating to fighting under gravity in confined spaces.
Which, again, was one of the reason mobile suits were developed, for close-quarter combat against capital ships and fighters. Moreover, there was the need for mobile suits to operate in the interiors of space colonies, in which their next closest rival were Earth Federation battle tanks, helicopters, mortars and infantry. Mostly thanks to the arms and legs, mobile suits could operate in pretty much any environment, which really came in handy when it came time to invade Earth and the Zeon found themselves outnumbered thousands to one.
Okay, I can see the benefit of "spreading out" your space APC eggbasket a bit. Perhaps it would make sense for individual soldiers to have their own vehicles, for stealth or saturation purposes. But it still wouldn't make much sense to compromise those vehicles by limiting them to mecha dimensions and functionalities
"Limiting" them? In the original Gundam series the average mobile suit was the size of an office building. The Zaku-II carried a machinegun with a caliber of 120mm just for starters. Even that was considered relatively primitive compared to the directed energy weapons of the actual Gundam, not to mention the mobile suit designs that came after.
