You can check complexity or username reuse client side. You could even block the n most common passwords clientside if you wanted to.

But I could just ignore/comprimise/modify your client and submit whatever I liked to the server.

What's the motivation of someone able to do that and interested in doing it?

Primarily to get around arbitrary password rules that do not enhance the security of the password but serve to weaken it, e.g. only use special characters from this list: !@#, or sorry your password is TOO LONG (?!)

That's true, but only to a point. You can actually server-side check username/password equality, and a not overly long list of other unwanted passwords. You just have to check each one.