Bias is always a problem here, so there's no perfect answer. It also depends on the audience -- the marketer's title, seniority, and job description. Yet thinking about marketers in a broad sense, here's a priority question that comes to mind: How much should they care about Twitter -- namely when considering it as a search engine?
Twitter's main use is for communicating; it's a hybrid of instant messaging and blogging (and it can bring out the worst of both worlds, with people publicly broadcasting notes that would have been unnecessary to even IM). Ann Handley of MarketingProfs provides a number of ways marketers are using Twitter to communicate, but marketers are lucky to reach a following of even a thousand people on Twitter; only a few tech brands tend to get that many, and there's no way to determine active readership. I'm skeptical of marketers using Twitter (and related services like Jaiku, recently acquired by Google) for communicating right now; that's a topic for a different column, though if you're interested, you'll find a rousing debate on my blog where a number of Twitter diehards tell me I have no idea what I'm talking about.
Twitter can be used for other purposes beyond communicating. Allen Stern on the Center Networks blog offers ideas on how Twitter (or other sites like StumbleUpon) could make it easier to find and discover new people and feeds (or tweets, if you want to use the lingo for Twitter messages). He has some great ideas, but he didn't take it far enough. One of the biggest problems with Twitter right now is that all you can do there is search for people, not content. There are two ways that's changing: from internal and external forces.
As for external forces, before Facebook made it cool to open up a social site to outside developers, Twitter established itself as a hotbed of innovation where third-party applications improved its service. Until recently, Twitter has had such a dearth of features that just about everything you'd want to do with it other than the basic posting and reading of feeds came from external tools. This is especially true for searching Twitter. On a rudimentary level, there's the Twittermap search engine, where you can enter any keyword and see what people are saying about it, but only over the previous twelve hours.
Twitter also is finally catching on itself. It now offers Twitter Tracking -- a way to subscribe to alerts much like Google News so you can find out when someone's tweeting about a brand or any subject. This is a must-try, regardless of whether it becomes part of your daily routine. If you want to store them, you can set up the alerts through Google Talk and have those IMs saved automatically in Gmail. For popular conversation topics like the show "Heroes" or Twitter itself, it feels like eavesdropping in a public place.
Just because customers are talking about a brand doesn't mean marketers need to listen. If someone stands on a street corner in Peoria and shouts that a national retailer's benefits stink, that's not an effective way for the customer to relay the message. If and when he convinces a group of people to join him on that corner, that's a nuisance, not a problem. But if the local news covers it, the Associated Press could pick it up, and suddenly it's a national news story.
This customer could alternatively use Twitter instead of the street corner. It's just as inefficient a way to communicate right now; he'll find a small audience, he'll be lucky if anyone else cares, and the gripe isn't going directly to the retailer. Yet with Twitter, it's possible to find that person before he finds an audience. And of course, there are positive scenarios, too; someone who wants to stand on his soapbox and rave about the retailer may welcome being the first to know about a new store opening, product line, or sale.
With the tools available, especially Twitter Tracking, it's now easy to monitor this channel with a minimal time investment. If it proves to be valuable, maybe that will motivate you to use Twitter in other ways. For most marketers, however, actively listening will put you a step ahead of most of your peers, and you can wait for Twitter to build its audience to do anything more with it.