I find that committing parts of a file in GitX is much easier than using interactive mode in the terminal. That's not a question of understanding the git model, it's just choosing the best tool for the job.
I completely agree. For many tasks the command line sequence of add, commit, push may be sufficient, but sometimes I don't care to wade through my editor merging and selecting portions of diffs to commit. The GitX interface is much more usable in that instance.