1) We don't want to reinvent the wheel. We are leveraging code that is actively developed, highly recommended and used by the web development community with open source licenses (GPL, BSD, AFL and MIT). We are not maintaining their code, we are simply bundling them into a package.
3) More Choices. If you think that dojo is better than YUI, you are free to use dojo. If you feel that there should be a wrapper for some very useful functions in dojo that are not in Ajax Helper, you can write them and help the next developer embarking on his/her next ajax weilding openacs web application.