Interestingly, I recently talked to someone in an interview setting who got asked a question about git, and as soon as the interviewee started saying, "Typically I use SourceTree," the interviewer's demeanor turned sour on asking any further version control questions.
Just because someone uses a GUI for version control doesn't mean they don't understand it... I prefer the way SourceTree makes it very easy and visual to stage portions of a file, and the always-displayed tree makes it very clear how different branching and merging operations are going to happen.
Seemed like some weird attitude against a GUI source control tool... especially one specifically well-designed for a Mercurial or Git workflow.