Yeah, I think it's worth it for some people. For example if you have a big monitor (1920x1200) and a smaller one (say, 1680x1050), the DS3P will allow you to have two video cards and let each of them handle one monitor. I hear that even an 8800 GTX struggles in this scenario if you don't have a cheaper card there to help it. I'll find out as soon as the Samsung 275T goes on sale
Plus the DS3P has FireWire, if you need it.
On the other hand, the DS3R has 3 PCI slots instead of 2. Very handy if you need to plug in an X-Fi, a TV tuner and a modem.