Is it Necessary For a Critic to Complete a Game? Absolutely Not
It's a common question:
Should video game critics complete the game before issuing a review for public consumption?
Having reviewed games for over 15 years, I can tell you without a moment's hesitation that the answer is "No." There are some rare instances where it would help a lot to finish a game, I will admit that. This is especially true for titles that thrive on the storyline, such as Heavy Rain. However, such titles are few and far between and in fact, they're actually becoming rarer as the industry embraces bigger, more open-ended experiences.
This isn't about laziness or arrogance. It's simply about producing an accurate, proper review and 99 percent of the time, you really don't need to see the end credits before you can do your job.
Mechanically, not much changes after the first hour or two
From a gameplay perspective, if the game is flawed, you're going to know it immediately. If it plays well, you'll also know it immediately. A game doesn't suddenly break or get significantly better in the last few hours of the adventure (at least in terms of mechanics). Now, if a game has multiple gameplay mechanics, you might have to see them all before writing the review; in other words, if there's a puzzle or platforming section in an action game, you'll want to at least get that far, so you can check out the new gameplay system. Developers do some things well and other things... not so well.
Other than that, there isn't much else to talk about. You know things are off in the first fifteen minutes of Yaiba: Ninja Gaiden Z, for instance, as the camera is poorly implemented and there are a few other control issues as well. On the flip side, you know The Last Of Us controls almost flawlessly right from the get-go. There are things you'll have to see, of course, such as melee attacking vs. third-person shooter elements, but you get a very good idea of the game's solidarity within the first hour or two.
Completing every game reviewed is a literal impossibility
It just can't be done. Unless you've got a very large staff and you get all the games as soon as humanly possible, this just can't happen. Gamers basically demand that reviews are produced within the first 24-48 hours of a new title's release; that being the case, critics have to really move to hit those deadlines. It's all the more difficult when the publisher doesn't get you the game until only a few days before the launch. Sometimes, you'll get games as much as two or three weeks ahead of time, but that's rare. And again, you have to be part of the biggest publications on earth for that to happen.
Let's also not forget that even the shortest games in existence are at least 5 or 6 hours long, with the majority being much, much longer. During last holiday season, for example, you can't expect a critic to complete Assassin's Creed IV: Black Flag in a matter of days, not when he or she has a dozen other games that need reviewing ASAP. What about upcoming titles like The Witcher 3: Wild Hunt? I mean, are you kidding? Either gamers have to be willing to wait a lot longer for reviews, or they just have to accept the fact that we don't need to complete the game in order to provide them with a great review.
A good critic knows when he's seen enough
That's just the truth. The longer you do it, the more you can easily recognize the point at which you can say, "Okay, I get it." At that moment, you know you can sit down and write a properly educated review. Maybe it's only a few hours; honestly, for some games, that's enough time. For others, you've got to put in a bigger chunk of time and you know it.
For the record - and I'd like to make this clarification now, lest people believe this practice encourages mistakes - I've never once felt like I screwed up. I mean, I've never written a review without finishing the game, gone back to finish, and then wanted to change the score. The most I've ever wanted to adjust is maybe a tenth of a point or two, but that's about it. The point is, I saw what I needed to see and I knew what gamers needed to know.
This is just the way things are. The idea that a critic has to complete a game in order to provide us with a great review is illogical and really, for most of us, completely impossible.