Please note that SQL Developer has included a Query Builder from a third-party since the 3.0 release. I will ping the developer that acts as liaison; perhaps you will get a more definitive response. Until then, you may consider the response to one bug I logged concerning some complex SQL the Query Builder's parser did not properly handle:
Query Builder is a visual environment targeted at novice/intermediate users. There are a number of advanced and/or hard to visualize language features that are not supported.*
Perhaps the use case of opening an external SQL file (potentially large / complex / with comments) may go beyond the intended usage of interactive development for the novice/intermediate user.