This fanpost is not arguing we should return Soriano to the leadoff spot. Rather, the point is that if you thought Soriano was bad as a lead-off hitter, Ryan Theriot is worse. This claim isn't based on the small sample of 2010 stats, but rather based on what we know about these hitters, without overlooking Soriano's disastrous 2009.
What do we want out of a lead-off hitter? First and foremost, we want a hitter who gets on base a lot to give the middle of the lineup RBI chances. Because he's on base before our best hitters, we want him to be a good base-runner, not just fast. Next, we want our lead-off hitter to create runs with his bat, because he'll be batting more than anyone else. It would also help if the lead-off hitter sees a lot of pitches to show the rest of the lineup the starter's repertoire and to get him laboring early. One of the complaints about Soriano as a lead-off hitter was his inconsistency. The argument is that a lead-off hitter should be counted on to have a professional approach day in and day out.
By this criteria, Theriot is a worse lead-off hitter than Soriano. As for consistency, in 2010, Theriot was a different kind of hitter month to month, but that may be on account of the power hitting experiment, which we shouldn't expect to be repeated. More to the point of consistency, note how he really struggled against fly ball pitchers and power pitchers, hitting .216/.287/.328 against the latter.
Last year, Theriot ranked 15th in the number of pitchers he saw per plate appearance, behind Soriano. He was 12th in runs created per 27 outs, again behind Soriano, despite Soriano's bad season. Over the past two seasons, Theriot was caught stealing on roughly a third of his attempted steals, whereas Soriano, since coming to the Cubs, has sported an excellent success rate at base-stealing.
The one thing Theriot projects to do better than Soriano is get on base. But Theriot's OBP projections top out in the low .350s, which is only slightly above Soriano's mark as Cub lead-off hitter. On his career, Theriot has hit .194/.286/.226 as the first batter in the game, and his OBP is only .326 when leading off an inning. On his career, Soriano has hit .310/.350/.620 as the first batter in the game.
The fact is that the Cubs don't have anyone who does all we want from a lead-off hitter. That fact makes it worth prioritizing what is most important from a lead-off hitter with a willingness to look for outside-the-box options.