I don't think it brings a lot of things in terms of enhancing actual game play or creation, which are the most important factors for me. GMS seems to primarily focus on portability (export to multiple platforms), integration (ads, game centres, etc.), standardisation and efficiency/optimisation. While they are all pretty important and substantial changes/additions, they're all predominantly external. My point is that I don't feel there would be a whole lot of difference between a game built in GM8 and one built in GMS. Nevertheless, I can create something in GM8 and then drop it into GMS, add some GMS features and export to another platform and be just as well off, if not better off due to having the benefit of both worlds. There are some things about GM8 that are just better, such as its portability, speed, human-readable error messages and the fact that it just works.