Again, StealthSigma, I didn't say they make you twitchy in SR4. I said the opposite. However, *if* we're talking about things making you twitchy, I'm asking why Synaptics and MBW should get a free pass when Wires has a specific rule.
Let's look at the physiological elements of the system that these three objects (Synaptic Booster, Wired Reflexes, Move-by-Wire) occupies and what must entail in the process of these augments that enables to function as they do.
You receive a stimulus of some sort, be it an attack or otherwise which transmits down your neural pathways to your brain. The brain then interprets it and sends the response down your neural pathways to your muscles to react to that stimulus.
Let's start with synaptic boosters and wired reflexes since they are just two different ways to accomplish the same effect, which is a faster controlled reaction or the ability to act faster. Both of these augments improve the neural pathways either by increasing the bandwidth and data rate of the neural channel (synaptic boosters) or by using adrenaline stimulation and boosters (wired reflexes) to achieve the effects. In both of these cases, your conscious is still retaining control over your motor responses. Wired Reflexes, in that light, is quite simple to turn off to avoid any potential jitteriness by just not running the adrenaline stimulation or neural boost. On the same accord, with both these augments, jitteriness should be able to be overcome with a relatively short learning time to acclimate yourself to your faster reaction times.
Move-by-Wire, on the other hand, uses an entirely different mechanism to achieve its goals. It completely replaces your ability to control your motor reflexes. Think of it like a RAID controller for computers. When you have one installed, the computer "sees" the RAID controller as a single hard drive. It doesn't see the individual hard drives connected to it. The computer cannot do anything to those hard drives, it can only send the hard drives operations to the RAID controller which then performs the appropriate tasks on the physical disks. Move-by-Wire occupies that same sort of spot. It controls your motor responses and can do so more efficiently than your brain can (as indicated by +2 Reaction +2 Dodge per rating rather than +1 Reaction). The jitter in this system is described in the fluff as a flaw in the controller mechanism using an incorrect correction to the seizure movement. It is not unlikely that in order to achieve the results of MBW, the neural connection to your brain must be severed for your muscles in order to place the controller. This is a good, functional, reason why MBW cannot be turned off. Turning it off would be tantamount to losing all motor control.