hmm I know for Halo Reach and Call of Duty Black Ops and MW3 you didn't need two gold accounts.
Borderlands 2 is completely different from those games.
no it's not. They're FPS. They may have different objectives when playing, but they're not completely different. Especially with Halo Reach, where there's co op.
edit: Also I believe Left4Dead allows guest as well.
Not all FPS are alike. With Borderlands, you're not going to be playing Team Deathmatch and the like as you would with those three. The 'campaign' also does not play the same as the Halo's or L4D's. And none of those games would really have a problem with having a guest playing, because there'd be no need to save their progress at all. Borderlands not saving the guest's data would cause problems. All that loot they picked up last session? Gone for good. Their class would also likely have to mirror that of the person they're playing with, so they'd be stuck playing the same character.
Borderlands just seems like it would have too many different problems with a guest account that your other examples would not. I just don't see it working well with Borerlands, or even happening. If I'm wrong, and Gearbox has it in and makes it work, good on them. --- "All down the church in midst of fire, the hellish monster flew, and, passing onward to the quire, he many people slew."
in borderlands 1 you could take a guest online with you...
Borderlands 1 has splitscreen, but no online splitscreen. I'm wondering where you got this idea. And I hope Borderlands 2 gets System-Link splitscreen, had some friends over to play borderlands and we found out you can only play splitscreen offline, so we had to rotate every few missions because we only had 2 Xbox's hooked up --- PSN/GT-Aadrianseth Borderlands 2. 9/18/12. Cant wait!