While I agree with and subscribe to the basic concept that indexes might not need to be defragmented to perform well and that rebuilding an index that is quickly fragmented can actually cause a bit of a slow down because it does remove splits that are like having a low fill factor in all the right places, there are larger problems associated with fragmented indexes... those same page and extent splits. They can take huge amounts of room on disk and in the database. While that won't affect the size of backups, it will affect the size a restore will require and it will affect the size of the mdf file on the database. There's a tradeoff to be had and you need to consider it carefully on the larger indexes.