As to why the library is still useful:
http://forums.wowace.com/showthread....110#post322110 I forgot to mention the Lib:IsUnitBeingRessed(unit) returns both boolean and resser, whereas the Blizzard API just returns the boolean, afaik.
Links. You can get to the localization tab of both from them.
http://www.wowace.com/addons/smartres2/
http://www.wowace.com/addons/librescomm-1-0/
tl;dr: The API does not return caster, only boolean for unit being ressed. Nor does it return cast time. Also, if your target is a unit frame or in the world frame, you get "unknown" and with the library, you always get a target, which is especially important if OTHER people want to know whom you are ressing. You need a comm for that information.