The fact that I prefer a GUI for source control management (Tower for Git and Versions for SVN) probably marks me as a lightweight, but I've found them to be valuable for 2 reasons:
1. I write better commit messages when I use them because they spellcheck and it just _seems_ like I ought to use proper punctuation and capitalization.
2. Having a one keystroke / button diff from the previous version offers a good sanity check to myself and helps me catch stupid errors from being committed to source control.
That being said, I still usually end up using a mix of the GUI and command line for some of the other features.