choice is evicted on Thursday night then it goes to #2. If the voting was from Thursday evening through Friday and then awarded on Sunday then they'd still get to use it that week (unless it was something like the veto replacement) and we wouldn't run the risk of voting for someone who is no longer qualified to receive it.
Basically, the fear comes from a situation like this: A concerted effort has been made to rally the fans of the outsiders to give their votes to Alex. Now Alex is on the block but she's not the target, in fact, when voting begins and throughout most of the week there's very little talk amongst the hgs to vote her out. Then, after the majority of the votes have been cast, the house has a last minute flip and Alex is voted out. Now fans of Alex and her "side" have already voted for Alex and there's little to no time to switch her votes to another hg on her team and instead, the temptation will likely go to someone on the other side. This wouldn't be the end of the world, but with a large alliance picking off the outsiders, having the outsider fans lose their chance to help an outsider while the other side wins another temptation could be seen as frustrating and unfair.
I see both sides of the issue though. Why vote for someone who is on the block after veto when the votes COULD flip? But then, look at Christmas winning the last temptation, she could have gone home and the next in line could have won. That's the chance voters have to take. Just like the hgs have to trust that their vote will be on the right side and they won't be left hanging with a hinky vote, viewers who vote need to understand that it's Big Brother and no one on the block is ever really safe so voting for them at all is a risk that could make your votes a waste of time.