But people could just create a new free account once they reach the 20-article limit. Perhaps they are hoping that readers will not be clever/motivated enough to clear cookies and re-register...

i'm glad someone else posted this. i do the same thing. i don't know why, but it annoys to no end to be forced to log in when i've clicked through to an article from somewhere. i'm sure that i am in the minority, and so my opinion does not count, but it really has made me realize just how little i care about reading some of the articles that a simple login dissuades me from actually reading them. i have a feeling that once the paywall limit is hit, far fewer people will then the nyt imagines will be tempted to pay. they may just close the tab and move on with their lives. that's fine, of course, the nytimes doesn't have to cater to people who don't value their product, but it does pave the way towards a more specialized, targeted product. people happily subscribe to news sites about minor league baseball or the franchise restaurant business. we'll see what happens here.

I expect they will. However its fair to say that the large number of people that use them will be eclipsed by the number who don't.

It is accepted and known that if people are determined to get around the paywall they'll find a way. For that reason its deliberately porous - a convenience factor or sorts.