graduate admissions - Is it unwise to contact the professor directly before getting admitted to a program in US?

In his answer of Details an applicant should include/exclude in an introductory letter to a prospective grad school adviser?, aeismail says

If the program in question is in the US, for instance, you should probably never contact the professor directly until after you've been admitted into the program in question. Since admissions decisions are handled centrally, it's just a waste of time.

Now I'm surprised with this, and I think I'm not the only one. For many times, not only in this site, but also in real-life examples in my university, I have seen that my friends get accepted through contacting professors beforehand, and those professors are helpful and willing to get you through the adcom. Recall it back, they don't go to US, but I don't think US should be an exception.

Why is it an exception? Why do the "admissions decisions are handled centrally", unlike other universities in the world, where the weight of the professors is heavier? Isn't that sorting the applications by score not good as checking their ability directly through interview? And who is the most suitable interviewer, if not the one who will advise you in the future?

Answer

Let me answer this question from the perspective of our school. If a school does things differently the answer you're quoting might be very appropriate, but not in our case.

Our admission decisions are made by our department, and essentially everyone is on the admissions committee - we share the load of evaluating applications. So, there could be great benefit of talking to a professor ahead of time. If one faculty member strongly desires a particular candidate, that student will generally get admitted, assuming they meet our qualifications. (Note that, as a small school, we have enough capacity to take more students than we do because we don't get tons of qualified applications. Also, we almost always have TA funding available for qualified students. But this really varies from one university to another.)

So, this establishes that, at some schools, talking to potential advisors ahead of time can be very valuable. The interest of working with a particular advisor can also (and should) go into the personal statement. I read so many personal statements that you can tell are completely generic, with different school names copy and pasted in. So, when someone says something specific about my research area, I take notice, and I think other faculty do too.

The real issue is that I get so many requests from random students to look at their CV. Many of them have very little connection to my work and are probably mass-emailing professors. This should be avoided.

@Anonymous' answer has the right idea here. A few months ago a student contacted me with a list of publications and a precise statement of interest about my work. We exchanged a few e-mails and I looked at his work. Another student tracked me down after a talk at a conference and talked to me in more detail about the work. These were both appropriate ways to contact me that may benefit the students in the future.