Midflight,
Winning the phone vote is a necessary but not sufficient condition for the bottom couple to be saved in the latter stages. Thus, whilst voting won't save your faves, not voting for them will see them in the DO.
If anyone is confused by Tess saying "Vote to save your favourites" then they don't deserve sympathy, they deserve to be locked up indefinitely for their own good. The basic principle can be demonstrated with the three couple, only one definitely saved scenario. There are only 6 possible outcomes, and it's not hard to determine that the bottom couple needs to win the phone vote and have other results go their way to avoid the DO. If the expectation is that the population as a whole can't work this out then there is really no hope for the country.
The obvious exception to this was in last year's semi when Tom could not be saved, irrespective of how anyone voted, and thus Tess urging people to vote to save their favourite was on decidedly dodgy ground. Even then, it took about 30 seconds to run through the options and conclude Tom was doomed.
The combination of stupidity and taking it all too seriously is very dangerous, IMHO.