The people I know with Plat. AMX (and Black) use them for the extras - being able to get into most of the airline VIP lounges for free, getting reservations in a booked restaurant, points (esp with something like the Starwood and Delta AMX cards). I don't know of any Visa/MC that offer the VIP club and restaurant booking? I've also heard with a black card they will bump someone out of a hotel if it's booked and you want to stay there.
Who cares what the interest is? Pay your cards in full when the bill comes in and you won't have to worry about it
I really don't care about the flash - my favorite card was my Bank1 Visa with pugs on it - then Chase bought them out and canceled it. It was a sad day :*(