Nnnnnhhhhhh, I put the second option in the poll, purely because there isn't an option for 'depends'. Which I think there should be. Thing is, some games it just isn't feasible, but some games it is. And it depends on the workload of the reviewer. It's like, if it's a relatively short game, or it's a game that's got a short story and then bulked out by other stuff (and I don't mean just multiplayer, I mean Fallout 3 for example, which has a fairly short story but is more than made up for with hours upon hours of sidequests), then I think the reviewer should make an effort to complete at least that story first. Unless they have a massive workload, like multiple games or if they have another job (like Yahtzee writes for PC Gamer, has the Mana Bar and a TV pilot, he has his own games he's allegedly working on, etc...).
I guess what I'm trying to say here is that when the reviewer has the time and energy and only has one or two games to worry about, then yes, they should make the effort to finish them. If they don't have that luxury, then they at least need to play through as much as possible simply to get the best possible look at the game and make an informed and fair judgement.
I for one, however, make it a habit to never trust any reviewer who doesn't put at least a couple of hours into the game. Same with people who only listen to half an album, or leave a film halfway through. If you simply can't be bothered to put in the required effort, then why is your opinion worth my time listening to?