When we started, our intention was to go sole. However, a number of prospects (which eventually turned into clients) indicated a preference to work with a Ltd company.
If we hadn't gone Ltd, we would not have got these contracts.
There is also the image argument (which I'm not getting into), where it's perceived that a Ltd Co. is somehow more professional than Fred Bloggs.
Not necessarily my opinion, but one I have heard on many occasions. Guess some of it depends on who your target market are.