Well, to us I think it would be irrelevant as it wouldn't take anything 1/2 as complicated or energy intensive to do in our planet. However, an FTL weapon could be relevant to an FTL capable species at war with another FTL species. What is interesting is that motion through space + motion through time must add up to the speed of light. So an FTL weapon would arrive at the target before it was launched if I understand the consequences right.