Overriding the right-click context menu in web browsers - pros and cons

We are programming a web application (not 'just' a web site, but functionality-wise a real application), and have the following discussion for the next release:

• our UI designer wants to replace the browser's right-click context menu (showing our own menu where appropriate, or no menu at all) because he wants the web app to be more like our (existing) Windows app
• our developers (and I) strongly object because this is bad practice, and simply something you do not do in a web application

Thus, I'm looking for "more solid" arguments - like best practice guidelines, any statements from reputable sources, coding arguments etc. - for the pros and cons of this issue, which I can hopefully use to resolve it once and for all...