1. Not a very good reason. Some projects might have slow CIs. Some projects might not have a CI at all. Some project's CI might not be checking everything (front end for example)..
2. Because people make mistakes. You might think you are only excluding a comment, but might be excluding something that is required by mistake.
If I really want to make sure, I do:
1. git stash
2. build + test
3. commit
4. git stash pop
> Some projects might not have a CI at all.
Well, then you have bigger problems. Without CI, how would you even know if your projects compiles on other platforms?