Continuity mistake: When the scrunt forces his way through the door, it opens the wrong direction. The scrunt was inside the building trying to get out. When the latch broke, the door opened inward instead of outward, as if forced from the outside. How could the scrunt have done that? (01:38:15 - 01:38:45)
Continuity mistake: When Cleveland introduces Mr. Farber to Young Soon Choi, Heep says her apartment is 8A. But, later when Cleveland goes to Choi, the door number reads 16B.
Continuity mistake: When Reggie is fighting back the green beast he is standing in very short grass. In the next shot he hears the great eagle coming and he looks towards the sky. When he looks back at the beast he is standing in very high grass which was never near him.
Continuity mistake: Before Cleveland dives into the water after Story, he takes his glasses off. However, when he gets out of the pool, his glasses appear on him again.
Continuity mistake: When Cleveland is sitting on the sofa and waiting to hear the bedtime story, he eats a cookie and takes one sip of milk. At first he only gets some milk caught in his mustache, but in the next shot there's a lot more milk there.
Answer: It's not a matter of symbolism, but of making the audience remember who he is. If Reggie was introduced as a normal guy in the beginning, with no significant feature, the audience may not recognize him when he's introduced as the Guardian. The working-out-one-arm thing makes him memorable enough to be instantly recognized again later when necessary.
It is not that it makes him memorable - It is like Mr. Heep says - that he wants to be special. "Every soul has a purpose." Reggie is striving to find his - only it finds him.