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Even without 51% of the mining power, if you have say 5% of mining power and you deliberately only mine 1 transaction in your block aren't you going to create a traffic jam of transactions and effectively DDOS the network? (And the 1 transaction could be a movement of the attackers own funds backward and forward).
Roughly 5% of blocks are going to get no useful transactions processed.
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With 5% of mining power you only mine 1 every 20 blocks. "The attacker" would somewhat slow the network down, but they wouldn't stop it from working. He would need to pay for all the harware he is using and he would deliberately loose the potential 0.5 BTC per block in fees.
If this is done at a time the network is already stressed, then this still appears to be a cost-effective attack. – AndyM – 2017-02-27T03:40:15.663
@AndyM Consider the fact, that it would drive fees the higher the more effective the attacker is, which would mean more hash power will appear and make the attack less effective. – Hubert Jasieniecki – 2017-03-02T18:41:57.983
The only transaction that a block must have is the coinbase, in order to claim the reward. Every other transaction is optional. – Nick ODell – 2017-02-24T20:27:33.247