While my primary focus is (and remains) Flash, I am an interactive developer. I would be sorely remiss to ignore other technologies. Not only does knowledge of other technologies potentially open new project opportunities, but it lets me reinvest ideas and mental models from them back into my work with Flash. Working with C# has already sparked some new ideas for me (not to mention a few AS3 feature requests). Knowledge of alternative technologies also lets us suggest the best possible solutions for our customers or talk them out of a bad one.
That thought should be extended to all technologies, including HTML5. As an interactive developer you have to be familiar with the technologies around you. The best part is that it’s a great way to get ideas for what you want in Flash/AS3 as Grant is finding with C#.