Tx fee prevents another type of attack (namely spamming the memory pool) or at least raises the cost. However there is no required tx fee so if there was no maximum block someone with sufficient mining power for example could make a 1TB block (all free txs made by the miner) and now the blockchain is >1TB.
It's extremely likely that a 1 TB block would be orphaned well before the miner transmitted to his first peer.