Well, yes it would weaken it, but then you would only have a solid non-maneuverable bar
. It shouldn't be too complex to replace if it breaks or malfunctions; we made a bar that rotated like you are describing last year out of modified 2" aluminum square tubing. It was light and durable and really easy to make replacements. Just remember that the arm is going to be most vulnerable at that joint that it rotates, so try to reinforce that as best you can. Also, torquing that arm in a direction perpendicular to its rotation is whats going to be your biggest concern with all these robots crashing into each other and entangling arms (at least thats what our problem was last year).