Flex Builder for vCenter-Plug-in development

Hello,

I'm currently using a trail version of Flex builder available for vCenter-Extension development. We don't you use Flex so often, Is there a requirement to buy Flex builder (Flash Builder 4.6) license for vCenter plugin-development. Wondering what are cost vs  benefits specifically for vCenter-Plug-in development that I can point out for justification to our company .

Some of the tools provided by VMware are pretty neat, such as vSphere-Web-Client project creation wizard which has Flex-Plugin . Does this have dependency on Flex/Flash builder ?

Thanks!