I think it was cruel to expect them to dance the showdance when they had just been knocked out the competition. A good showdance needs a healthy dose of adrenalin to get it going and with Lisa and Brendan, having just been knocked out the final - even though they had scored the best marks with the judges, which much have added to the disappointment - the adrenalin just wasn't there.
Even without that, they were at a disadvantage as Lisa is so tall. Swinging her about above Brendan's head, just wasn't going to be an option, so they had to come up with something else. It was a good concept, and had it worked, it would have been fine.