I was on a boat trip recently and another photog and I had the same kit: 7D and 100-400 IS.
I was puzzled to see that he kept his on-board flash firing all the time (photos of seals etc. from boat).
Didn't like to ask him why he had it firing - this was on a very sunny day and the seals were at least 20 feet away.
I thought focus assist was used with an external flash & generally used when in dark conditions to help the camera focus? or am I missing something?