Using a thrown tube as a defensive object to interfere with the opponent attempting to score is allowed within the rules, as is using a robot as a defensive object, within limits. Is it a good strategy? that is debatable. Is it ungracious? I don't think so. As I have stated, this is a competition, you are supposed to COMPETE. If is allowed in the rules, it is OK to do, and therefore cannot be ungracious.
In my opinion, it is dangerous (at Peachtree, on two occasions, teams received red cards for trying this strategy and de-scoring tubes). It was attempted several times on our team, and we just welcomed the tubes and scored them (except the one that deflated after a particularly hard throw at our bot).
Here is a video of a HP trying it. Watch at about 38 seconds in.