In the situation that an FO is passed over for command due to lack of qualifications they still hold their spot on the seniority list, so when they do eventually make Captain they jump ahead of all the Captains who skipped them in the first place.
In some cases it may actually work in your favour, if for example you don't have the required minimums for command and then work your way to the top of the FO list you could bid for anything (Days off, duties, etc) you wanted and probably always get it. Also you would be last to be called in of reserve. When you finally did make Captain instead of going to the bottom of the Captain pile you would slot in above all the junior guys who jumped you in the first place, avoiding the reserve call ins and crap duties there as well.
