The only negative thing I could really see coming out of this is if someone is having a technical issue, a mod may frequently come to be the first answer. This is good for people who are willing to use mods, but if there is an unmodded way to fix the problem, that's the answer I would like first.
As I've noticed twallan himself answered with non-mod answers to technical questions, so we shouldn't have to much fear about people offering up mod solutions to problems. This community has a large group of knowledgeable people and will offer up mod and non-mod solutions so you have a choice.
I don't think the community will change dramatically because of the addition of 'mod talk'. Seems many of the members here were mod users anyways and couldn't be honest with their stories, with their gaming, and with the other members here because of the rule. I do think there should be a "mod list that Carl's Sims 3 Guide approves of" So people know where to go first if interested in mods. The rule should be reworked to still include "mods should be used at your own risk" because this is no different than downloading anything else off the internet (whether its mods, CC, buildings, or worlds). Also including "mod talk should only be used on appropriate forum boards, please refrain from using it elsewhere" would help keep it off the general boards.
Also some people assume mods are only there to "fix" bugs, while this is true with many mods, some also enhance the game play making it more fun, or cleaning up problems that make games almost unplayable. But ultimately the choice is up to the user, and I think a message board to help people make an educated decisions is great. After all isn't that what Carl's all about? Helping people?