"With GPS updating, the comments above are very valid. However, for aircraft relying on DME/DME updating, there is merit in a full alignment, time permitting, on every turnaround."
Although mentioned, I don't think enough emphasis has been placed on things other simple "position" when it comes to IRS alignment. If pilots start believing that lat/long (position) is the only thing to consider, you will eventually get the situation where pilots of GPS-equipped aircraft consider it safe to take off with the IRSs still in the _middle_ of a quick alignment... thinking that the IRS's will align themselves in the air c/o the GPS (It has already happened).
The reason why many manuals state that a full alignment must be made if the IRS's have not had a full alignment for 18hours (irrespective of whether the aircraft has GPS or not) is because not all types of IRS errors are removed during fast alignment.
Remember also that if triple mix IRS position varies by more than a certain amount from GPS position (due to IRS alignment errors), that many aircraft are designed to default to IRS position (irrespective of whether the IRS's are correct or not).
Rgds.
Q.