How can people not understand that Jobs was just comparing Apple's phone to Android/WIN phones? Fact is phones drop bars when the antenna is interfered with. SIMPLE! By giving a demo on it he's just saying "look, if you're gonna make a big deal about this then be fair and give everyone a hard time!".
Exactly! The issue is not that the other phones are bad, with better or worse antenna designs; the issue is that it is an antenna and the laws of physics make it impossible to design an antenna which cannot be interfered with.
Apple's new design is apparently more sensitive to radio signals than their older designs when it's not being interfered with, but it's also apparently easier to interfere with it than most phones, and interference makes a bigger difference in signal strength than on most phones.
By showing what happens to some other phones when they're held different ways, he was trying to point out that this happens to all phones to some degree.
I should also point out that phones with the antenna at the top are not blocked by the hand so much, but it also puts the antenna right next to your head. Apparently the FCC has been encouraging manufacturers to put the antenna at the bottom of the phone to reduce the amount of radiation being pumped into people's skulls, which moves the antenna to an area where holding the phone in your hand can interfere with the antenna.
Since the bumpers can apparently reduce the amount of interference without significantly reducing the sensitivity to radio signals, Apple apparently has room to further improve the antenna design.