One can see why Marr asked the question: if Brown was on anti-depressants that could affect his judgement then the public has a right to know. It wasn’t in the public interest, as Andrew Marr seemed to acknowledge in his interview with George Osborne, that the BBC sat on the story of Charles Kennedy’s drinking. Also, one can argue that once the rumour is out there it needs to be dealt with.
Burt on the other hand, I wonder if Brown should have been asked such a question without some more evidence for it to be based on. Even though, Brown said that he wasn’t on any pills the mere fact of him being asked if he was will have an effect on how voters view him.