I hadn't thought of it but I guess having a head start on the next block in larger pools can help, but Eligius is consistently pool #2 or #3. I wonder how much difference it would make over time compared to #1.
I'm not sure. Technically you can't hold the answer for long; if someone else gets it you totally lose. But if you have huge hashing it might be worth it, I don't know. Then again I am probably missing something very very basic, anyone?
One thing large pools have over smaller pools is they are more likely to be able to build on their own block, hence reducing orphans, since the longest chain wins.