I've had a press badge for several conventions and as you state they do have advantages and disadvantages. Most of the big cons require you represent some publication or present examples of articles you've authored.
Kintoki-Con was the only con I have attended that offered a "Photographer" badge. I think you just had to provide a link to your convention work to get one. I thought it was really great of them to recognize the value good cosplay photographers bring to a con. The headlining band used several of my photos in their promos back in Japan so it was a win-win for everyone.
I do agree that with a press badge you feel more "on-the-clock" then without. IMO, if covering the guest's of honor, bands, panels, masquerade, etc is important then it's worth the press badge. If you're mainly there for hallway/casual/photoshoots then not so much.