I'm not quite sure... Many of us use resistance bands or DB's in order to increase our punching power/speed. We will box several rounds which are much harder with the added resistance. We overshoot so that the real thing is easier. This same logic should apply to the case the OP gives. If he struggles to keep his hands up over the course of several rounds, then using weighted gloves would make holding up unweighted gloves so much easier. He won't be able to hold up his gloves nearly as long but through training with them the real thing should become so much easier.

So either one of us is totally wrong or I misunderstood the OP