This is what blogs are about, eh?
The issue of value of the job talk, a blogosphere phenom  initiated by Dan Nexon has now escalated to the usually rarefied policy precincts of Dan Drezner , so rather than work on the [long] list of things I was going to do as soon as I got into the office, I will briefly [for me] chime in.
Specifically, on the side of the pro-job-talk faction.
The political science academic job market has many flaws, but the job talk is not central to these, nor is it completely artificial.
1. Organizationally, it is the one time most of the faculty see the candidate at the same time and have a common base of comparison. Furthermore, since usually the candidate will present on a topic fairly close to those reasons he or she has been interviewed in the first place, it is relevant information.
2. As several people have noted, the key part of the talk is not the presentation—though I have, in fact, seen people screw up at the presentation level, albeit usually with a research design that has some stunningly obvious flaw—but the questions, and these can be very revealing. The frequency with which the candidate lost the position—either outright or, more commonly, someone else simply did a better job: remember, you may not have done anything wrong, rather someone else may have done better —due to the questions is probably two or three times higher than the next most frequent reason.
The University of Kansas has—or had: things may have changed—an absolutely devastating approach to candidate questions. A certain category of candidates typically come to interview at Kansas expecting to find cows grazing in front of the [small] library and a barely literate faculty—they did not, apparently, bother to check where the faculty had obtained their degrees. So the candidates would tend to be caught off-guard by the level of the questions and—I will use the technical term here—start to bullshit an answer.
At many institutions, the response to such activity is that a few designated pit bulls, either senior faculty who do this sort of thing when they aren’t engaged in alternative amusements such as pulling wings off flies, tossing kittens into wood chippers and the like, or the junior faculty desperate to stomp someone into the ground in hopes this will somehow qualify them for tenure, go after the candidate, questioning their training, their overall cognitive ability, the civil status of the candidate’s parents at the point the candidate was conceived, and similar issues. These rants typically go on for some time, giving the candidate a breather, who then lets the awkward topic die out with some comment like “Well, yes, I suppose that’s another theoretical perspective/nuanced social construction…” and goes on to something else. Whew.
That’s not what we would do at Kansas. When the candidate started bullshitting, we’d quietly nod and ask them to elaborate, and see just how far they’d go before they’d realize they were making a complete fool of themselves.
Life can be cruel.
3. The job talk is not an artificial exercise. Someone who cannot put together a carefully rehearsed one-off presentation for 45 minutes in their area of specialization is very unlikely to be able to pull off six to nine hours a week of newly-prepared lectures for fourteen consecutive weeks. And teaching pays the bills.
Forward through time and there are a surprising number of occasions where someone is presenting something that looks a lot like a job talk. To take an extreme example—and of course I’m very proud of this, and I’m not pretending this is typical—I was once asked to give a talk to a very high level audience—a couple of two-stars and one three-star, as I recall, plus a scientific advisory committee —at the Command and General Staff College at Ft. Leavenworth, and was told it would be twenty minutes. The speaker following me had a flight delay and I ended up talking for more than hour, essentially doing a review of quantitative approaches to political analysis. Extreme example, but it can happen.
4. In my experience, the quality of contemporary job talks borders on the intimidating, the sort of thing that leaves senior faculty saying “I’m glad I’m not on the market.” Most of those I saw by candidates at both Kansas and Penn State have been extraordinarily polished and professional, and the standards have risen substantially over the past two decades. At Penn State, no student goes out for an interview without at least two full-scale practice talks, and if it takes more, we do more. Based on the quality of most talks we see, I assume that other programs do the same, and if you are in a program that doesn’t, you are at a serious disadvantage.
The system is not perfect: obviously some people are simply better presenters than others , native (or fluent) speakers of English have a clear edge, and I’ve known some cases where individuals are highly qualified but were sufficiently introverted that they didn’t make a good impression . But I don’t see an obvious alternative, and I don’t think the existing system is that dysfunctional.
Back to dealing with that enthusiastic R&R from APSR. 
1. Pronounced “feeeee-nom”
2. Who also has the other relevant blogy links, which I won’t copy at the moment.
3. We call this “life.”
4. #2: Making it clear to the assistant professors and graduate students that you are way too qualified for the position and the department is lucky you even came out for the interview. Such folks usually end up as baristas, though their stories are long repeated in departmental lore.
5. You get a quirky little medallion for this, the same sort they used to give Native American leaders at the end of treaty talks on the annexation of, say, South Dakota.
6. I’m guessing debate experience helps. Improvisational theater experience—formal or just street theater—probably really helps. Seriously.
7. Class background also can be a factor. Albeit in my favorite instance of this, the individual—a former pipefitter—went on to a highly successful nonacademic political science career and more than once has been courted by institutions which would not give him the time of day while he was on the market to hire their students.
8. A joke, a joke!!!
9. Those wolf-like creatures chasing the dwarfs in Hobbit: The Movie?—those are the assistant professors. The orcs?—those are the vengeful fulls. You?—probably Thorin after about the third time he’s whacked by Azog’s mace.