Allow me to be the oddball out here, but there's more to shortening services than just shortening your URL. Services like Bit.ly and Tr.im allowed users to actually monitor the traffic the goes through the links. If you're a news source or advertising company, wouldn't you like to know how many people are clicking your links. My local news stations and papers all use bit.ly and they are able to see how many people actually click on a link, which would be important to them.
Though knowing the creators of Tr.im actually stated "No business we approached wanted to purchase tr.im for even a minor amount" just goes to show how questionable the services actually are during the current economy. Its a shame that the developers had to lose money for a convenient service, but the economy is having a hard time supporting new businesses, especially where there is already a market holder (in this case Bit.ly).
As far as being a bad idea, it's not. 6 months down the road, you'll have found a new URL shortening service and been using that. Yes, if people go back 6 months looking at your posts they won't work, but odds are if people do a search for the topic they'll find out right now. Usually the trends are slightly more interesting and have some correlation to the news, but it would appear right now it's an odd time... so whenever the more "newsworthy" stuff gets to the top then this'll apply: usually if you search Google News you will be able to find out why something is trending... it's not always the case, but down the road if someone sees your post on Michael Jackson, Google Voice, or iPhone they can just go to Google News search for the topic, set the date range and voila... you know what the was a twitter. *gasping for air* Sorry, horrible run on sentence there....