I'm for it. And, as Tangerineduel has noted, we should also have one for Iris. And if there were a snowball effect and you did one for all the companions, I personally wouldn't have a problem. I think the old categorical guideline applies: as long as your category has (the potential for) three members, then it can be created. Now, I'm not saying that every 3-member notion should be put into a category, but as we've no objection to the "planets visited by the Doctor" categories — and because there's no graceful way to collect this data through an SMW template — I don't have a problem with "planets visited by <whoever>" for any long-serving recurring character, really.