Fix for CVE-2008-1447 changed internal socket management logic which caused that named needs more sockets simulateously opened than before. System call select() can't handle such many opened descriptors.
When server load increases approximately to 1000 queries per second then messages like "too many open file descriptors" appear in the log and named fails to handle next incomming queries.
Internal socket infrastructure has been reworked. System call select() is no longer used in favor of epoll infrastructure.
BIND is able to handle far more bigger number of queries simulateously.
For complete list of bug fixes check /usr/share/doc/bind-*/CHANGES