Hehe, went through all the maths on this stuff couple weeks ago on other threads. The scoring isn't the same as the Tom situation last year, cos there is always mathematically a way that Chris can avoid a dance-off - but it depends on the positions the judges place Ricky and Ali relative to the positions the public vote.
When they ridiculously allow themselves to get down to a two couples final, the problem from a producer's POV is a different one ... you can't guarantee the public will at least get their favourite to a final under the current system. With three couples, you can.
So if semi-final comes and contestant Z is getting 60%+ of the public vote, for example, and the judges manage to score in a way that Z ends up in the dance-off and they kick him/her out, you just found yourself in the dumbest position the producer of any reality tv show can be in. Your judges just stuck two fingers up to your audience, said sorry you're not having your favourite in the final ... oh, and btw, tune in next week to watch the two people you like a lot less dance the final. Ratings disaster - not only for one series, but for future ones, as a good section of your audience comes to the realisation their vote does not necessarily influence anything. They can't even get their favourite to the final.
If they stick with two couples final, easiest way round this problem for production is to drop a DO for the semi-final ... judges scores + public scores then top two straight through will see the public favourite always come at least second.