I have surveyed journalists articles and what players have said about the event since 1970 (I posted the article references on this forum couple of years ago) - and what is very clear, is that the players historically have viewed it as only a little lower than a slam win eg
http://news.bbc.co.uk/1/hi/sport/tennis/539395.stm
eg Regarding the WTF final in 1999 between Sampras and Agassi
"the last major tennis final of the millennium"
http://www.atpworldtour.com/News/Ten...Qualifies.aspx
Or Berdych
For me, it’s always one of my goals at the beginning of each season and it’s an honour to qualify,” said Berdych. “It’s just great to be a part of it. It really is the celebration of tennis, only for the Top 8 guys. It’s a huge show which I suppose tennis should be, and for us tennis players I would say it’s the best tournament.”
These are just a few of the many quotes available showing journalists and players alike view it as a very important tournament.
(For a time in the late 70's and 1980's it was regarded as a defacto major). Hence, it's current value of 3/4s of a Slam for an unbeaten run at the WTF seems about right to me. So if Slams get valued up from their present 2 x Masters 1000's then the WTF should remain at 3/4s of a Slam.