Because it is WAAAAAY better quality. I believe it is 10 phase compared to 8 for the P6T.
4.5 is stretching it for 24/7, possible, but not probable, on the Classified. The CPU has a lot to do with it too. I can get 4.5 on my eVGA x58 SLI LE that was cheaper than both of those. Cooling the CPU at 4.5 usually requires at least water. Unless you have one hell of a chip, 1.40-1.45 volts minimum for 4.5. Some CPU's would require 1.5 or more and you can almost forget that on water too. I can do 4.5 at 1.45 and my temps were approaching 90c and I'm on water, with two 3x120 mm rads.
The P6T will throttle at about 4.3 unless you flash to a special BIOS for the Deluxe version 1. Without this BIOS, 4.5 on any P6T is unlikely if you stick with a 21x multiplier. Without the 21x, impossible on the P6T. That would be a 225 BCLK. The Classified is one of the few boards that can possibly go that high.
If you got the bucks to blow, go for it. If you are looking at it because you think it will guarantee a 4.5 ghz 920, I wouldn't, especially if you are only using one video card. This would be complete overkill.