the_traveler gives a good explanation of the only logical reason I've heard for the gates and the kindergarten walk... WAS, NYP and CHI have small platforms. I can completely understand why Amtrak does not want a platform packed full of impatient passengers waiting for a Northeast Regional train... and having passengers who are arriving on said having to fight their way onto the platform.
Personally, I prefer to sit down in the station until it's time to board, instead of standing on the platform to wait for my train. I'd just prefer that Amtrak get rid the gates and simply make a boarding call... saying that passengers can now board train xxx on track xxx.
While that explanation makes some sense, those of us who recall the pre-9/11 situation remember that things worked pretty well without the gate naz.., er, gate "dragons," at least for the corridor trains. At NYP, yes, they didn't let you on the platform until they announced the train, and I agree that the platforms there are kind of narrow, but they also didn't bother with the ticket check, because most of the Northeast Direct/Northeast Regionals were unreserved until 2003 or 2004, and even for the few that were reserved, I guess they figured they could throw you off the train at Newark if you didn't have the right ticket.
At WAS, even though they didn't open the gate until the train was ready, no one cared if you went on the platform to take a look. In 1993, I was at the station and walked out on the platform where the Montrealer was boarding to take a look at the consist, and no one bothered me at all. Also, they used to let MARC passengers congregate trackside at Gate A. This helped relieve congestion in the concourse, especially if there was a MARC service disruption. It would also help if they used the interior boarding lounges in gates B-D, which are usually empty, while the cattle line for the regionals snakes outside through the concourse. In fact, for me, the biggest benefit of AGR Select Plus isn't just the points, it's being able to go into the Club Acela and bypass the cattle lines.
I can't speak as much for CHI, because before I went to this past Gathering, the only time I've traveled from CHI was on a long-distance train, but, yes, the platforms are pretty narrow there, and the diesel fumes do accumulate under the trainshed. But the cattle lines are really, really bad, and sitting in the Metropolitan Lounge doesn't help you get around them.
In fact, these three stations are all similar in that they're terminal stations, and the real problem might be that the trains aren't ready to board early enough to disperse the crowds waiting in line. The MARC trains are usually ready for boarding at least 15 minutes before departure, sometimes they don't board regionals at WAS until 10 or even 5 minutes before departure.
At my home station, BAL, they usually don't do a ticket check (though I saw them doing it last week during the holiday rush), nad you can sit in the concourse until the train is announced. On the other hand, if you want to go down to the platform to railfan, nobody stops you. Also, when they first announce the train, that means it's 5-10 minutes out of the station. If the weather's nice, I'll go down to the platform, but this time of year (or in the middle of the summer), I think I'd wait until the final call.