But you'd also have to ascertain how much it was their second choice to be of any worth. One person may be 100% in favour of their first preference while only 5% in favour of their second, for another it might be 60%/50%, should that be regarded as the same thing?
Which is why it'd be best to ask, surely? That's what AV does
That's a bit of a leap. They may not have wanted them most, but it doesn't necessarily follow they positively didn't want them.
If a government gets in on 30% of the vote, it means 70% of people didn't want them. So you end up with a government that most people didn't want in the first place.