JoeGKushner
Adventurer
I do not see why, if they have changed a setting in a way that is not to ones liking that one is in anyway obliged to follow then in the change.
The info in the older books is largely system independant and can still be used.
Depends on where you look.
If you're running the Realms in 4e using the 4e, then no, the information was rendered pretty useless. The massive advance in the time line took care of that. It was one of the things that even with the Avatar crisis, you could kind of gloss over in a lot of areas because the timeline advancement wasn't that big. Ditto for 3rd ed.
Now if you're looking at 3rd ed FR books, they are heavy in crunch. That crunch is now fairly useless and a lot of the things that were unique about the Realms havent' been updated to 4e and probably, at the rate they're going, never will.
The spells, magic items, and monsters of the Realms, had a lot of life, detail, and quirks. 4e doesn't handle the later very well, although it is proving capable of handling the former.