Well if they use your private members thats all in theirs responsibility. What if they just want to make the thing work, ship the product, whatever else, and don't care about new versions.
In other situations you don't have to listen, but if a popular library doesn't pay attention to backward compatibility then they'll end up in a situation where major customers keep using an older version of their library and nobody wants to upgrade. And unless you want to keep maintaining multiple versions, you want them to upgrade.