The advantage is differences in sound. Like I said, I'm using ENGL and VHT both are known for having their distinct sounds. So I'm mixing two companies sounds. Your Recto will always sound like a Recto. My set up sounds like and ENGL through a VHT lol. Also there are tons of options with having multiple preamps and power amps. Let's say I want a Marshall sound, a Mesa sound, and an ENGL sound...I can get a Marshall JMP-1, a Mesa Triaxis, and have the ENGL 530 I have...then just use one power amp and set it up so I can switch from one preamp to another with MIDI. Seperate preamp/power amp set ups are all about flexability and versatility. Also, most tube power amps are stereo and most rack effects units have stereo effects. Some people really like the way stereo effects sound. I know I do. I also have the option with my power amp of running one side of my power amp in class A and the other in class A/B or both in A or both in A/B.
Versatility is really the key word when it comes to making a rack set up with separate preamps and power amps.