Knowing at least the basics of major platforms is always a good idea, if no for other reason than to appreciate what other approaches and possibilities are out there. Your career is not going to consist of Flex until the day you retire - Flex will be going away eventually, just like
Java will be going away. What's more, Flex doesn't run on iOS at all, and it doesn't run particularly well on Android (it's certainly not a big player) - and those platforms have seen massive adoption in the last few years, and are now used beyond just phones. So -since you need to continue learning anyway- you might as well learn something popular.