I think the problem was that they only had two and a half minutes. 'Swingathon' implies 'marathon'. In the time available, it was impossible to get a sense of any of the couples (and for the judges to get a real sense of them too). If they'd had to dance for even five minutes, we'd all have seen what they could (or couldn't) do. But it's usually assumed that we, the viewers, have the attention span of a gnat, hence this. And yes, it would've been much fairer to have a pro dance between the end of Matt's and the beginning of the Swing-thing.