I'd like this. It would be good for FCs with coil groups but extra people that can't go because the active members aren't in groups of 8. We would be able to get them some experience through out all the phases and point out which mechanics they have to look out for when they have to pug it (so sad). SE can even go the extra mile and keep score for each fight and tell them how many times they would have died from a mechanic if they actually fought the same way in normal mode coil.
My FC has 1 coil group and some extra members, we probably could train the extra members for coil but since the static group's schedule keeps clashing, we could only go to coil once or twice a week so we have to tackle as much as we can in one go. So if we go to coil with extra members outside of the static, we can only go to turn 6 and NOT win it. They're left pugging it and some have just stopped playing because they'd rather not (all they have outside coil is dailies and weekly cap).