Scrappy,
It's not that the thickness has that big an effect on the dwell time (at least that's not what I was thinking about) :)
It's more that a very light covering of developer, one that falls naturally, rather than painted on with an aerosol can, will help the more sensitive inspections draw out and show small or tight surface flaws.
If a thick coating of developer is applied (especially with FPI) there can be very small defects that are covered or will not bleed out sufficiently to be seen, even with a black light.) If the indication is small enough, all the dwell time in the world won't make it appear if the developer is misapplied.
Now dwell time is important, don't get me wrong... Thats why people smarter than me develop those procedures, with dwell time depending on the amount of sensitivity required, with an eye toward the size of discontinuity that can be accepted... For example; if the inspection criteria calls for a minimum size of 1/8" for defects, it makes little sense to create a procedure sensitive enough to find much smaller indications..