Marketing and PR are out of control
In some ways, I can't blame Marketing and PR people for wanting to control everything. They're asked to look over something that's out of their control to begin with: How people think and feel about their company and it's products. Not to mention, in the Enterprise there's a ton of employees and simple tasks are owned by committee. It's no reason Marketing people yip-yip-yip like chihuahuas when anyone else touches the powerpoint template.
Marketing and PR people are like IT 
The IT Department is asked to make sure that everything in the company is secure, so IT's default answer is "no" unless execs tell them it's ok. Marketing people are asked to influence people's feelings inside and outside the company so their default answer is "we have to do it, we're the experts" (so, another form of "no"). Then, a group of Marketers work with some agencies to whitewash everything to look and feel the same and Marketing then thinks they've done their job. I should know, I've worked in those departments.
You can't stop people from talking about you
You actually want them to talk about you, right? Yet, invisibility or apathy is the effect that constant Marketing "no" has on company's brands. There's no need to guard, no one wants to attack. Regardless, the truth is that Marketing is overburdened. It's stupid to task them to be responsible for molding a big Marketplace to their little will. The world doesn't work that way. If a product sucks, it sucks and no amount of Marketing can fix that. It's just as stupid to turn them into defensive Brand cops, who enforce martial law on any communication that goes out the door without their approval. Unless it's a confusion around copyright, or a hairy PR situation, there's nothing to defend.
You are not a beautiful and unique snowflake. You are the same decaying organic matter as everyone else, and we are all part of the same compost pile. -Fight Club
Get permission to move from transaction to interaction 
Marketing needs to be released from being solely responsible for changing perceptions or driving leads. They should be enabling the organization to make meaningful, positive customer experiences and connections. This may seem like a subtle shift but when Marketing can feel comfortable becoming listeners instead of blasting sales messages, dramatic change ensues. Suddenly, employees start to really learn about what interests the market without a commercial agenda. Real conversations begin and Marketers begin to enlist the assets of the organization. This results in much more positive customer experiences. I think of this as ROB ("Return on Behavior") others may think of it as some form of Net Promotor Score (NPS).
Meantime, back at the Jive ranch, we get shot down daily
We have tons of customers who don't want us to even mention them. We want them to talk about the cool stuff they're doing. Or let us promote it. But they always say no. Why? Because "it's just our policy" or "we have to protect the Brand." Keep in mind, companies using our software are doing really positive things. They're either uniting their company and improving the way they collaborate with each other or launching publicly accessible online communities where they're actually listening to their customers. These are leadership activities and both of which increase the ROB/NPS.
Results of my Twitterpoll
Questions I asked:
- "Why do Marketing/PR departments stop others from talking about them?"
- "Why do companies think they need to guard their brand?"
- "What's an example of when not guarding a brand has resulted in something bad?"
"Most are interested in telling their story, not genuinely interested in listening to customers. I was talking with a friend last night about models of abundance versus scarcity - most Marcom is based on scarcity and control. The going-forward model is tapping the collective (social web) and abundance of thought to drive improvement."
Jonathan Yarmis, AMR Research (@jyarmis)
"One could argue that Microsoft, by not protecting the brand, went from plucky underdog (even while big) to convicted monopolist. Microsoft didn't protect its brand and in the vacuum they got defined by third parties more heavily. You know that's not it but they didn't fully see how the discussion was going, i.e., they didn't manage even what they could."
Jeff Mann, Gartner (@jeffmann)
All brands which through overuse became synonyms for an object rather than a brand. Kleenex Bandaid Hoover
Steve Mann, SAP (@stevemann)
I only think of competitive reasons why not too share. No reason around the brand unless they are close to your space Old habits die hard (but its an ugly death).