I agree. The vast, vast, vast majority of threads here are not plus threads, and within the first four pages (and I'm being generous) of most threads we see that they end up being threadjacked into something else by people that are arguing with the premise or who wish to turn it into The Same Argument They Have In Every Thread(tm).
The reason that "+" threads were explicitly allowed by the mods is for those occasions when people are looking for collaborative discussion around a topic, or for those occasions when people want to discuss something without the topic being threadjacked by particular known issues.
Given the relative paucity of "+" threads, and the very large number of threads in which we can argue endlessly over everything, I don't think that the existence of "+" threads is an issue.