The problem they have here is that while in theory Harrold isn't necessary, 7th D exist to step in if there's an injury - I wouldn't expect Harrold to get more than 20 games even if he is kept, he'd function more as an 8th defenseman. Keeping Urbom up as a 7th doesn't make much sense to me, unless they're convinced he's ready and they'll swap him and Volchenkov from time to time and then he'll be the first guy in with an injury. This doesn't seem to make sense either.
Indeed, Lou said he's not comfortable with someone making an NHL salary in the AHL, so that seems to make it even clearer.
It's not so much any of this making sense, it's just a way to protect Urbom from waivers. But it's unlikely that Harrold gets claimed anyway. Lou worked himself in a corner, they just have too many one ways and they've made too many signings. They may be able to hide Tedenby for now, and hope they can play a shell game to prolong it, but Urbom's going to be tough. It's possible he doesn't get claimed because he hasn't played much at this level, but you never know. At least if they trade him, there wasn't that much of an initial investment, but Lou will look a little goofy pawning him off for nothing after pumping his tires for so long.