It's a weight and stress issue on the tower. Pulling a tube from the tower puts a load of side load upon it that it wouldn't ordinarily get. In fact almost all mfgrs tell you not to and some will void warrenties on the tower and any gel damage incurred. I know a guy that did it for the same reason and cracked a tower weld on his new boat, factory said nope.

While some will argue that it equals the weight of a boarder, the boarder offers a lot less resistence and that's what cracks them or creates stress cracks.