This question has come up recently for me. It is, indeed, a valid question and has been in the back of my mind for a while now. Why play a retro-clone over the original game of Dungeons & Dragons when you still own the original OOP rules or can purchase them online?
For me, the Retro Clones brought back an excitement and gaming philosophy that lead me into the game in the first place. The revisions, presentations and community of these 'new' rules pulled me in.
It would be a tough argument to say that the original OD&D rules were written in a clear, concise manner. There was a lot of ambiguity and contradiction in those original rules. I'm not saying that that was a terrible thing but instead of just clarifying those original rules, they were rewritten into the 1ed Players Handbook and Dungeon Masters Guide, which in my opinion, was a complication of the game and lead us down the path to the rules-intense versions of the game that are out today. Holmes did a pretty good job of editing the OD&D game into something that was an presentation improvement in his 'basic' version of the game.
Now, I felt that Moldvay's basic rules improved upon the layout and explication of the Holmes rules but I always felt the writing was geared more towards children. Even though I was young at the time, I still read from the Holmes rules, more because of the way it was written than anything else. I didn't need the writing to be 'dumbed' down, I needed the rules to be clarified.
The retro-clones do just that. They are well written 'revisions' of the original rules keeping the original philosophies and concepts of the original intentions of the game alive. Things are left open enough for the GM and players to adapt the game to their vision and style.
Sure I can still play using my original rules and you can play those rules how you like, but I also feel that Retro Clones are a good way to step out of the weight of what D&D was and had became without having to get into any arguments or issues. My players have all played different versions of the games from Holmes basic all the way up through 4ed, but now we're all just playing S&W. No arguing, no expectations just good gaming fun.
Many version of the Clones can be obtained as free PDF downloads. You and your players don't have to spend a lot of money buying volume upon volume of rules just to sit down and play a game. That, alone, can bring new players into classic RPGs. And with WOTC pulling their pdf files of the OOP material, obtaining copies of those publications can be difficult and expensive.
I feel the game was meant to be fun, quick-paced and concise and these various rules do just that. It's really all just that same game but the clones have gotten me back into playing the game once more. And we've got 6 players now and more want to play. Anything that brings new players in or old players back is a good thing as far as I'm concerned.