get a clue
Let's say you want to display graphics on the fly. Say something about stock valuations, with the exact question being specified by the user at run time. I am not talking about the simple stuff you see everyday, more the result of mathematical equations on stock ratios. You can easily generate an SVG snippet with the correct data to provide fairly complex information. You are in charge, along with what SVG supports.
Now, if you are using Flex (or Silverlight which I have not used), you will have to code to their graphing APIs. You are now dependent on exactly what capabilities Flex bakes into their graph rendering engine. Additionally, I know the Flex Eclipse plugin is fairly cheap, when you buy it _without_ graphing. Not so with graphing.
Flex and Silverlight may be the answer to coding rich internet applications - I found Flex very nice. They are not to generating free-form information on the fly. And, if there were identical capabilities, in terms of result and, relatively, ease of programming, in HTML5, supported by most browsers (ha ha) then I would still prefer to code in HTML5 over more "proprietary" solutions.
But software is rarely an all or nothing world except for fanbois.