I think depression is a very broad name for many different things. Depression, as a whole should not be counted as a disability. It depends on how it effects you, your responses to it and the causes of it.
If it is due to a chemical inbalance which cannot be solved, then it should be recognised. I still would not go as far as to say recognised as a disability, but recognised as a problem. If somebody is confined to their home due to their depression, then it should also be recognised. This severity would also, in my mind, lead to help with benefits and disability aid etc. but not necessarily recognised as a genuine disability.
Where the line is drawn has to be the most common form. People who have depression, without the chemical inbalance from birth who are still able to do normal life things. I'm sounding harsh here, so let me elaborate. Nobody would willingly let themselves to be confined to their homes without a genuine problem, and nobody can prove a chemical inbalance which isn't there. What would worry me if it were classed as a disability would be people saying they have it as an easy way out. I'm not saying anybody does that at this point. I just don't think it would allow for any incentive to help themselves. I've known about 4 or 5 friends who have had depression: 4 of them have said that all the help is awful, but they do not want to get better. They admit they like the attention. I've only had 1 friend who has truly helped themselves and wanted to get better, and she went to the same therapist, same group and said that everything given helped. This may just be my friends, but still, it seems certain people don't want to get better, and I think by giving extra support in that way would not truly help. People need true incentive to get better.
Ps. I am aware incentive would come due to depression not being pleasant, obviously.