It really depends on why you're playing an RPG in the first place. If you see it primarily as a storytelling medium, then you probably won't see anything diminishing in playing a character weaker than others, and you won't be inclined to abuse a character more powerful than others. If, on the other hand, you see RPGs as a competitive medium, more like a game of Diplomacy or Battletech, you're unlikely to want to play a character who doesn't match up in power to the other characters, and giving you a character with more power than most is a bad idea. Most players tend to fluctuate between these two extremes--RPG as a cooperative exercise versus RPG as a competitive exercise--and this tension is part of what creates the particularly unique dynamic of the medium.
I myself think that the important thing is that everyone feels they're making some kind of contribution to the story of the campaign. No one should feel like a bit player or fifth wheel. The usual practice of beginning (and, generally, working to keep) all player characters at roughly the same level/point value/whatever is simply a quick and easy way of making this happen.
John's use of other people (myself as Nanami, Keiko, Juri-Av and Motoko, Phil as Corrine, Merc as Gabrielle, Brem as Gideon and Lian, Mike as Ukyou... sorry if I missed anyone) to run characters who essentially function as NPCs lends an interesting perspective to this. In my own case, I've run characters in John's games who were considerably more powerful than the PCs (Nanami and Juri-Av), considerably less powerful (Keiko), and equally powerful (Motoko)--speaking in all these cases in terms of pure point value.
In large part I think this approach of John's works because of the different nature of the means by which we conduct our RPGs--through the medium of MUCK or IRC as opposed to face-to-face gaming. Because of the convenience of drifting in or out of the gaming as necessary, people are willing to play supplementary roles in a way they simply wouldn't be able to in face-to-face gaming.
I personally have my doubts as to whether or not a traditional "party" of characters of considerably different power levels could work out successfully. Dan cites Bilbo and Gandalf in Tolkien as an example of two characters with very different power levels, but Gandalf is a distant figure who seldom shows his true power--he would be an NPC in any traditionally-structured game, not a PC. Put his power into the hands of a PC and he will almost inevitably dominate the action to a degree that begins to quickly marginalize weaker characters.