Because every so many time I make a backup of those attachments. OK, so I could tell TB to save all those attachments in a certain place, but I suppose the attachments also stay in the emails as well, effectively doubling the harddrive space these attachments occupy ?
Call me picky, but I don't like that.

But I also realise it's probably only because I'm used to it being done in a different way for years now though...