What's the value of WTF title compared to a slam title?
I'd say player with 3 WTFs and 0 slams is as great as a player with 1 slam and 0 WTFs. But once you have slam titles, then a slam is equivalent to two WTFs.
It's really rare to win the WTF and not win a slam. Only three players have done this once each - Corretja in 1998, Nalbandian in 2005, and Davydenko in 2009.