For two reasons, one being that otherwise we'll only have a weekend to vote for the winner and two in order to salvage the deserving housemates for the actual final. There's no way the likes of Sam and Sophie will be up if it's normal noms, and with so few left it'll probably be people like Dexter and Gina (so long as he doesn't go).
With six people left after Friday, open the vote to win lines and the person with the fewest votes can leave next Friday, leaving five for the final. Perhaps even do another surprise eviction on the Sunday or Monday following that to leave us with a final four (a la BB6).
This way we'd most likely get a Gina, Dexter (if he survives), Charlie, Jack & Joe (not perfect but oh well), Sam (unless they did a four personal final as I suggested) final line-up, which whilst it isn't perfect, is a lot better than some of the other prospects if the likes of Gina and Dexter, or even Charlie were to go up again next week.
So yeah, open the VTW lines and let's have a proper 10 day-ish battle
I'd imagine the leader may change a few times during the week as well