Basically, any command that I have to even passively think about the parameters which is used often enough gets its own alias. This does not include commands like "git add" because I don't even think about parameters when I'm typing it out. Having to remember that I set ga to "git add" adds more work than I'm doing now.
I am willing to believe that aliases like 'ga' and 'gp' would actually increase productivity if all you did all day long was manipulate a git repository or you had some physical disability in which it was actually difficult to type on a keyboard, but I mean, using my source control tool is such a small piece of the larger initiative I take on day in and day out, that in the end, the extra keystrokes aren't going to have any meaningful impact on the amount of value I add to the projects I work on.
There's also the issue with portability. You have to be able to bring those aliases with you anywhere you go. A great example I heard about was at a meetup once where a guy tried to give a talk on git and because he wasn't using his machine, that talk was almost worthless because he kept typing his precious little aliases and, arguably, didn't really know how to use the tool he was trying to demonstrate.
Nor will removing those keystrokes have a negative impact on the value you add.
Fear of forgetting the original commands is a reasonable argument against using aliases. It's very similar to the argument against using vim plugins for fear that you won't know how to use vi on servers or other peoples' machines. Nonetheless, most people use the plugins on their development machines which they feel make them the most productive in their day to day work. Same thing for aliases.
Using vim plugins on your development machine should only add value to your standard commands. You still use standard vim commands to move around and manipulate text. And noone expects you to modify whole application on server, so you don't need those extra plugins for project manipulation, code completion (you still can type, right?) or highlighting