If a discussion ever gets to the point where people are talking in circles, I'm not much a fan of that. But a lot of the time this isn't really the 'fault' of the proposal, as people can be stubborn and it's really hard for someone to change their mind about something even when the discussion seems to indicate that their way of thinking isn't the most well-informed (an opinion can never be flat-out wrong, because an opinion isn't even fact in the first place).
I will add in though, that I've seen a lot of 'proposals' (usually the ones that end up being moved to this section) that are more just somebody ranting about a particular part of the game they had a bad experience with, rather than a traditional proposal.
However, discussion in the forums should be encouraged if the above is not happening. If you personally feel like the discussion's gone on for 'too long' and others do not, nobody's compelling you to participate any further.