I think the key word is "legal", I have been considering getting a 7 wood to replace my 3 iron ... I know when I bought my set (80's) there was a debate about the 7 wood, I was going to ask our Pro if it was legal (and to be safe, I still might).  My current Driver is about 8 years old, so I was going to get a new Ping Driver and figured I'd order a 7 wood at the same time.   If it is allowed, and it improves your game, I see nothing wrong.