If you want it for the setting, it's worth it. At least the FRCS. Either that or download all the FR stuff WotC has put for free on its site; and adapt the rules yourself.
Magic of Faerûn, Monsters of Faerûn, Lords of Darkness, Faith & Pantheons and Silver Marches can also be worth it as long as you can see an use for them (don't bother with Silver Marches if you plan on playing in the Shining South, or don't bother with Lords of Darkness if you want to use your own villains, for example).
If you want it for the rules, and plan to use it for another setting, it's worth it also as long as you think you can use most of these rules. Most of the material is pretty generic and/or easily adaptable, as it has been shown by seeing the FRCS feats in OA (as "ancestor feats") and in, IIRC, Wheel of Time. Two other above-mentionned books are also quite worth it, by decreasing order Monsters (the most generic), Magic (mainly spells and magic items usable anywhere), and finally, a long mile behind, Faiths and Pantheons (the prestige classes may be adapted to other religions -- it's a bit weak, but that's still more rule material than found in LoD and SM combined).
If by "Forgotten Realms", you mean each of the FR books plus Monsters of Faerûn, then the answer is probably no., however. Take the FRCS and Magic of Faerûn for a start, and then see for yourself if you may need the others.