More and more applications are moving to the cloud: Google Docs for productivity apps, Meebo for instant messaging, Gmail for e-mails, Salesforce for CRM, etc.
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Mac don't have a "right mouse button", likewise with a lot of touch screen phones etc.
Even on a windows application most normal users (not programmers or power users) don’t think to right clicking when they wish to do something, if they have not learned it off by heart to do the given task they wish to do.
(Also right clicking on most web pages brings up a menu that a normal user does not understand, so they don’t try it more then once.)
So you always have to provide another way of doing the operation anyway.