Uptime
Support
Illusion they are getting more

I think hurr durr is a proper answer.

Trust. (which has in some cases been earned many times over)

I can say that slush has worked through some real world challenges over a long period of time.

I voluntarily pay btcmine.com the same 2 percent as slush has in his pool.  I feel 2 percent is fair.

Well, whether you agree with them or not, those are 4 very real reasons that prevented me from switching from deepbit before the recent downtime.

The thing that annoys me with the BTCmine interface is that I cannot easily see how much BTC I made in the last 24 hours.  The best I will be able to do on that front is take an average of the payouts I've had vs the time it took to get them, but that isn't going to be exact for a 24 hour period.

"Free market principles" assume people are rational, which is why they fail most of the time in practice.

From a purely selfish point of view, calculating only what makes me the most money, I still often use a fee-based pool because of the following:

--The pool might have SMS alerts if a miner is down. Yeah, I can run my own program to do this, but the computer that runs that program and also mines needs to have a remote, preferably pool-side alert system regarding it's miner. The fees of the pool, easily, could be less money than the money I would lose due to unnoticed downtime. This is especially true for miners behind shitty internet connections and routers or modems that need perpetual reboots.

--The server's traffic volume, geographic location and it's long-polling settings contribute to the amount of rejected results that a miner gets. A no-fee pool that gives me 4% stales results in less earnings than a pool with a 2% fee and less than 1% stales (or that pays for stales).

--Because, right now, the market is bullish, a 3% fee with an instant payout could benefit me if I believe that the exchange rate will increase more than 3% in the payout time difference between that pool and another, delayed one.

--Knowing that pool-hoppers are out there, possibly gaining 90% more income while screwing everyone else over of, say, 2-10%, a pool that has a fee but is not a good pool-hopping target could potentially be more profitable for me, as I don't hop pools.

--A pool with a fee, but that provides good statistics can help me optimize miners, especially if they are on remote machines.

Depending on your priorities, there are many reasons to use a pool with a fee even if you act out of pure self-interest for your mining income. I often use one pool for one miner and one pool for others just because their needs are different.

This is a good point as well, and something I have considered to a great extent.  It's another reason I like the larger pools.  Less variance means you can more easily compare daily BTC rates.