Pro Simple installation and updates
A user doesn't have to be particularly technical to benefit from oh-my-zsh. You run a simple command to install and it asks you if it may check for git updates on a regular basis. You give up a bit of control but you get a tremendous benefit for very little individual effort.
Pro Updates over Git
This provides a robust update mechanism with full version control. That means that no custom mechanism needs to be implemented for upgrades of the user profile. It also means that your own modifications can be preserved while still allowing upgrades. And it means that you can downgrade at any time to any previous revision if anything doesn't work right with the new version.