It's not only that the security can be avoided (especially by foreigners "playing dumb" when called over in Chinese), it's that they are only checking bags. For those in Beijing who've witnessed the process from the time the security checks were originally put in place, it's well known that these checks have gotten stricter as we get closer to the Games, but there are still a lot of holes. The glaring one is that they don't actually check what an individual has on his person or any liquids they may be carrying. They aren't going to install airport style metal detectors that every individual must go through, nor do they test liquids, so all they are doing is forcing would be terrorists to move any weapons from their bag to their person.
With the threat of terrorism supposedly very real, these "security checks" do little to make me feel safer on the subway. Either get serious about security or give it up, none of this half-assedness that we've come to expect from the government. Terrorists are usually more committed to their attack then police are at stopping them, but the "security checks" in place are only for show and will do little to deter or stop a real terrorist.