The glitching is caused by the current draw of your servo relative to what your ESC and receiver can provide.

MM's stuff = known to be glitchy w/o a BEC and external or cap
Spektrums stuff = known to be glitchy w/o an external BEC or cap
They just don't provide a good, constant BEC to the servo.

The real miracle is that with those combination of products, you have a servo that works without anything added in.

There's no harm in running the cap, especially since most good servos tend to glitch off the MM especially, you'd be looking at a new ESC as well.