Reasons why his burning down a library might be bad:

• Risk of fire spreading (but does not appear to have been the case)
• Loss of private property by the owners of the building or books (but does not apply here)
• Loss of useful knowledge (does not appear to apply here, but disputed by JoshuaZ)

Seems you're missing a key one. Probably the most important one. At the very least, this is to me the primary advantage of even building libraries at all:

• Loss of a major means of spreading, disseminating and finding knowledge, whether the knowledge itself is lost or not.

Imagine trying to find information regarding a specific species of bird (a standard "Encyclopedia" will only have an entry on birds, not on each known species), with no internet and no libraries. You're going to run around for a while until you finally find someone who knows someone who's heard of someone who owns a book that might contain the information you want on that particular bird.

Libraries are, first and foremost, a convenient place to store a lot of books, which implies a convenient place to find any book in particular you're looking for with much higher success rates than asking a random friend.