I'll concede the perceivable value of a thread like that in the respect that it lets people vent, but I don't think it has the sort of value you think it does. Having been in numerous threads like that (and that's another issue, I'm sorry, but even if it has some value, it doesn't have "lets post the same thread 10 times and all circle jerk them" value) I don't see people coming up with ideas or trying to take the complaints and discuss solutions. The fact that the thread itself starts on a negative tone and mentality leads the discussion to be a lot of flaming, a lot of complaining, and a lot of insulting. I wouldn't mind the creation of these threads if it lead into something constructive even if that was negative toned. The fact is they don't though and that is why I don't think they have any place in this forum. I don't think instituting a guideline that requires players to make their post in a constructive manner, or at least construct the thread in a manner that prompts discussion, not witch hunting and flaming, threatens players out of expressing themselves. Personally I just feel it requires them to take the effort to actually construct something worth reading. Why should the community, or Riot for that matter, value anything that happens in a thread full of bashing and negativity? If a player can't be bothered to take the time to construct a meaningful and thought out thread or post, then I don't believe their feedback has much value, nor should it.
From my perspective its a lot like the following (simplified a bit for obvious reasons, there is a lot of grey territory here):
Person A and Person B (A, B) both have a bad game they lost because of an AFK.
A makes a thread titled "I'm really tired of damn AFKs, what do we do about it?" Inside the thread his post is a sob story about how he lost the game from an AFK and lost some LP. He complains about losing, fails to actually mention how he played or what he could improve, but does ask about a new system for AFKs and asks for ideas and suggestions. This is a QQ thread and a sob story, but Person A constructs it in a manner that lets them complain and vent, while still inciting a discussion around a fix or solution.
B makes a thread titled "Fuck Rito and theyre Shit Game!!!!11!!". Inside the thread the player flames the AFK, blames the AFK for everything and then insults Riot and the player saying they deserved that win and they demand their LP for the game back. This is a QQ thread that is clearly negative and offers no basis for a constructive discussion or sharing of ideas. This player may feel better after making the post, but they do so in a manner that is inappropriate and shouldn't be allowed in this community.
To me, this rule change is about encouraging Person B to not post what they are posting, because honestly, I don't think they deserve the catharsis they get from it. I don't see anything wrong with requiring them to post in a slightly more constructive manner like Person A, despite Person A having some negative feelings and complaining in their thread. The guideline is to distinguish between what is overly negative and inappropriate to post here and to help players construct a negative thread in a more discussion focused manner, so they can vent their feelings while also provide some ideas and valuable feedback.