You dont need to know xml if you want to work with plain text only, in ajax.
It should be worth the time, because, esp. you know javascript, you will be able to jump in in no time.
Thanks to books like Ajax for dummies(if your backend is php), and ajax in practice (if
java backend).
The issues you mentioned, e.g. browser standards - were here even before ajax came out anyway, thanks microsoft, so its not new.