The biggest selling points are probably the developer community, the ease of console support, and the larger track record of commercially successful games. Plus if you are a beginner the resources that come with a commercial tool can make a huge difference.
In a perfect world I'd just recommend the open source thing to everyone, but if I knew an artist who wanted to make a game I'm not sure that would be good advice.
Gamemaker is still focused on 2D. I also think its easier for people without programming experience and the design guides you more but is less flexible.