"Efficiency" and "arbitrage" are two separate things. "Efficiency" has to do with information. "Arbitrage" is based upon what people do with that information.
There is also a difference between riskless arbitrage, and risk arbitrage. Most arbitrage is risk arbitrage. How the different players value the risks, and transaction costs, determines if they are willing to do trades to close the spread.
So, even in a 100% efficient market, there are opportunities for risk arbitrage. Otherwise the big guns on Wall St. wouldn't spend millions of dollars to make all the pennies (on what you would consider is a very efficient market) that add up to millions of dollars.
The rise of multi-pools has led to a more balanced market, not necessarily a more "efficient" one. The rise of multi-pools is a simple effect of economics - supply and demand. Not of arbitragers. Someone is willing to pay more (demand), and the multi-pools have arisen to satisfy that demand (supply).
Very well said. Though I would not exactly call the financial markets perfectly efficient, and those guys wearing $10,000 suits on wall street doing risk arbitrage aren't exactly playing with all their own money. (And some of them are even working with better than perfect information in violation of the law because the US justice system currently does not punish thieves on wall street as harshly as they do someone stealing food to feed their family.). If you were to disagree with me on the level of efficiency within financial markets, I would refer you to this article I read in the Wall Street journal last month.
http://online.wsj.com/news/articles/SB10001424052702303947904579340711424615716Very briefly, the article illustrates how in any market, however well established, there are almost always opportunities to exploit, and in this specific case, speed to information. High-speed traders have moved from custom-built fiber-optic cables, to microwave, then millimeter-wave transmissions, in order to gain exploitable benefits over other market participants. In the old days it was market spreads, having priced securities in fractional values. Since moving to decimals, it is currently speed. Tomorrow, it will be something else.
But an equivalent rate of "earning pennies on millions" was not exactly the type of additional return that miners here will even notice. That would equate to only a 0.0001% increase in BTC/1MHD, and for most miners here, except you know who, that is not exactly going to make much of a difference.
Update: Or more than 0.0001%, depending upon how many times the process would be repeated during the course of a day. But still most here would not be able to notice the difference. And then if you had the formula to earn that rate via arbitrage, would you share the rewards with the pool or simply start an alternate business for yourself?