Makes it easy to build EDSLs. Very often, in an EDSL you want your users to be able to use a language that's almost like the host language, but has one or two key constructs redefined.

I'm not sure it's the choice I would've made, but I can see it being very attractive for someone whose primary experience with programming is designing a programming language. Back when I was interested in PL design in college, I was a big fan of this property in languages as well.