Al of Black Gate asks a very good question, How badly do I need another D&D?
The short answer is you don't, given the assumption that most of you reading this are playing some form of D&D or at least another RPG.
However if you are a publisher you may need another D&D very badly. And that what sets up the conflict. As Bob Bledsaw of Judges Guild, Mayfair Games, John Adams of Brave Halfling and Joseph Goodman of Goodman Games found out, when you don't have clear control over the core rule set your efforts are at the mercy of other people.
Unlike Judges Guild and Mayfair Games, recent publishers have more options in regards to how to deal with the situation thanks to open gaming. Namely the option of creating your own RPG. If you look at most of the big d20 companies still going to day they used the d20 SRD to develop their own system (Mutant & Mastermind, Castles & Crusades, True20, etc).
The catch is that once you branch out you are faced with growing your audience. And for some this is a challenge they rise too. For others it is one that cause them to crash and burn.
There is no right or wrong with this situation only that that choices have various consequences. If you are going to publish a ruleset be prepared to grow your audience and that the synergy of supporting the original edition is going to be lessened.
As for my personal view, I don't really want to try to grow an audience for a ruleset. While I like complex or interesting rule systems, I don't like designing them. The only reason I came out with a rules supplement in the first place because I been gaming for 30 years and managed to come up with a useful rule every other year or so. So it added up. I marvel at guys, like Clash at Better Mousetrap Games, who can sit down and whip an entire system, multiple times.
So when I started writing for RPG, I resolved not to write my own retro clone. Since that my view slightly modified, only because I get tired of having to flip through both my MW Supplement and the SW Core book. So at some point I may do a combined player's handbook but it very low on the priority list.