One benefit of being a separate project is that it can be used with non-Ruby wrappers. See node-sass https://github.com/andrew/node-sass as an example.
Since I'd guess most people interested in the node version will want the grunt one.
From what I understand there are some limitations to the implementation, but I haven't encountered any so far.
Instead of Compass, I've switched to using Bourbon (bourbon.io) but may consider just using Auto Prefixer in future dev efforts.
Source maps I can live without since I haven't gotten used to them. I know it's in the works or at least being thought about: https://github.com/hcatlin/libsass/issues/122.