"when the subject matter is esoteric enough, the GPLed library is sometimes the only good implementation" thats exactly when the GPL makes sense for a library.
For those who are in the business of promoting free software (such as the FSF) the lesser GPL is primarly useful when GPLing a library would only encourage gratuitous incompatibility and not encourage the GPLing of more software. ... Such as is the case when the functionality the library provides is widely duplicated.
(If you don't agree with the goal, you won't like this reasoning but this is not an unreasonable position)