The biggest problem is the cable companies. Many broadcast both a HD and SD version of local channels.
For instance, they take our HD signal, and pass it for the HD version. For the SD version, they center-cut our 16:9 signal, down-convert, and broadcast that as the SD version. We as broadcasters don't get a choice. We'd rather them letterbox the down-converted version, but they don't.
While I don't have to keep things 4:3 safe, I do 95% of the time. Though, every now and then, just depending on the circumstances, I put some titles or graphics out into 16:9 safe. Not critical information, just something like a logo or something. When I do this, we simply call it "added-value" for 16:9 viewers. 
I wish we didn't have to be 4:3 safe, but our hands are tied. Thank you cable companies!
Though, I do wish Adobe would add 4:3 margins in Premiere, just like After Effects does. Heck, our cameras even do it.
