OKay, to answer your questions:
first of all, when you draw large transients of power from a PSU, you'll cause a voltage drop no matter what even if small. WHen you have a seperate PSU for each amp, the amps haven't got much ease in interfering with eachother. Essentially, one amplifier causes a voltage drop on a bass transient or something whil the other is trying to produce vocals, the vocals on the other amp get reduced in amplitude at each transient and this can become noticeable. Basically, it's an easy way to get away with minimal size supplies and still give each amp less disturbance in the line.
For home theater it really depends on what your willing to pay for or listen to. You can achieve exceptional results with only one transformer for a pair or more of amps, but this transformer and it's supply capacitors mustn't let transient voltage drops occur.
Most dolby and DTS recordings have very very bad distortion figures on transients anyway so it doesn't matter too much with home theater. And try not to believe in THX labels either, it's a publicity stunt.
BTW, my next set of amps will run off of the same PSU and trannie and deliver: 2X600W into 4ohms, 2X100W into 8ohms, and 1X80W into 16ohms all from a 5kva transformer but I'm going to use 1 farad supply caps to eliminate transient problems. As you may see, it isn't a necessity to have seperate PSU's, it just a luxury if you can afford it...
Well... many limiting factors do occur in amplifiers and also, there are many ways of rating output power. In the Harman/Kardon industry, they often made huge understatements in the ways of power. Also, Harman/Kardon amplifiers are very very well designed for high current capability and excellent current handling. They will not drop off or flake out like many amplifiers these days when you demand bass from them. Thus, they seem way more powerful than the usual amp of the same rating. One good example of HK current abilities is that I could run my amp into a set of 4ohm and 8ohm speakers in parallel which results in a total load impedance of 2.6 ohms. I could crank the amp until it's gain stage distorted and the output transistors didn't even get hot, while if you did this to a modern home reciever or amp, it would turn into a nice fireworks show!