Disclaimer: I made GitUp for that very reason ;)
If you want to see side-by side diffs that's also a lot better in a nice text editor than in a text window. Getting balloon tooltips when hovering commit hashes hidden in a gutter etc. allows more information to be fit onto the screen.
The reason you use a graphical UI is because a text ui doesn't fit all the information you want to show in a good way. Most git tools might not need show or draw a large amount of information (push/pull/commit/...) but some do (blame, log --graph for example).
The one exception in my case is 3-way merges. I handle those in Meld, which I find to be better suited than any of the integrated threesome tools in these git GUIs.
Even that I could probably do better in emacs if I set it up.
At least, that's how I see it.
Also, I'm sure there must be a PPA or something for gitg on Ubuntu, if you really like the tool.