Well, you run the risk if being told your "request" is completely inappropriate and to return to your seat and not to bother other people with your selfishness, although most (but probably not all) FA's would dress up the wording.

There are hundreds, if not thousands, of threads on FT complaining about pax who do not sit in their assigned seats. The OP has absolutely no information on why the pax remained in his assigned seat which happened to be the middle seat; as long as the other person remained within the boundaries of his own seat, the OP is receiving exactly what his ticket entitled him to - transportation in one coach seat of his own - and really doesn't have any right to demand, or request that somebody else move just to increase his comfort. Getting the FA involved to move somebody else so you get the extra space is wrong.