Honestly, I think there are times when the use of a thread as a conversation is fine. if it's on topic and relevant, then there's (theoreticaly) no problem if it's just between two people.

if the topic itself is something moronic, that can be a different story, though

"If I wanted to read Wuthering Heights, I'd shoot my self."

I think it's fine if the topic is between two people, if it's something that might be of interest to other people. For example, if two people are having a conversation about creationism vs. evolution, and the posts consist of just those two people talking back and forth, it doesn't mean that other people aren't interested, they may just not feel they have anything to add to the debate at the moment.

On the other hand, when two people are flirting with each other, nobody cares except those two people.

Cat Clash

Amen!

Friends don't let friends take up scrapbooking!

What Mowgli described is what was actually happening, although it is not the only kind of flirting that is a problem.

Also, regarding conversations that are back and forth between only two people, there is an element of time/speed involved. In, for example, the question thread, if you answer the moment the other has posted, you are preventing anyone else joining in. However, if your answers had a significant amount of time between them (what constitutes significant varies from thread to thread), that would be different, because other people would have been given a chance to respond.

Mrs G
The Garner who cares.

okay, so we need to include something about this in the community rules.

how about "Attention seeking behavior that flies in the face of the established culture is not just a bad idea, it's bad manners. If people complain you're flirting or hyping your own issues excessively on the boards, you might be politely asked to stop. Please feel free to take that as a strict order instead of a request."

...

okay, so the phrasing needs some work.

"If I wanted to read Wuthering Heights, I'd shoot my self."