To take a different line...
maybe the problem was the 'expert' rather than the tools.
Doing a word frequency breakdown on the various leaders responses does tell is something about their approaches, if you just interpret the information you're given in the right way.
let's take just the first word Rowlett picked out for each leader.
'got' for Brown - Rowlett reckoned this was a mistake. No it wasn't, it tells us that Brown likes to emphasise what we have, what we've 'got', i.e. what he believes Labour has given us, and also indicates that he likes to use the imperative a lot 'we have got to do this' - or so he believes.
'country' for Cameron - he likes to view issues from a national rather than international angle.
'old' for Clegg - he wants us to do away with the 'old' parties, the 'old' system and he kept emphasising that
The piece didn't work because Rowlett didn't put any effort into it, not because there is anything inherently wrong with word frequency analysis (and a tag cloud is simply a visual representation of that).