Earlier quoted context omitted.
Zero transaction fees? The bitcoin network costs something like $3.5 million per day to run, and optimistically handles tens of millions in transactions. That's a transaction fee of 10%. You just don't notice it because they pay the miners by printing money. I'll be really interested to see what happens when they stop that and start relying on up front transaction fees to fund the network.
This is an excellent point. I'm curious to see how that pans out. My guess is that since Bitcoin transaction fees are supposed to be set by the market, they'll stay competitive. If mining is unprofitable at the rates credit cards are offering, fewer people will mine, which makes mining cost less energy, leaving more profit. Eventually an equilibrium will be reached.
If 90% of the network is powered down to reduce transaction fees, that gives someone the opportunity to buy up 20% of it on the cheap and corrupt the network. Bitcoin has to use as many resources as possible to stay secure.