GPL doesn't require you to distribute your server-side code if you are hosting it on your own servers (AGPL does). The client side code does have to be open sourced since it is distributed but Javascript code is pretty worthless without the server-side API, HTML and CSS that goes with it.
In other words, all the commercial license does from a business point of view is allow you to sue people who copy your client side code. That's a pretty poor incentive since most businesses don't even care about people stealing their Javascript.
In addition, as mentioned in the article, the GPL license makes people feel like they are contributing because they legally have to rather than because they want to give back to the community, which is not a good thing from a psychological point of view[0].
FWIW, I would advise the Meteor team to pick a more permissive license and reconsider their business model. Perhaps they can provide consulting, hosting, job board, certification, etc.?