Do you know that definitely with The Prodigy as that's sh*t if so. And likewise I wouldn't see them doing both, don't think it's ever happened.
Though as said before Kasabian did do Kendal, Y Not and Victorious, it's rare for it to happen one does all 3.
I doubt very few people will spend a lot of money on tickets to see the undercard alone unless you are the sort of people who hang around on efestivals. Yeah, if you have family, care duties of course you will put their needs first and not be that bothered by always seeing the last acts.
The special guest afternoon slot is genius and as you say works well for families but I don't think clipping a few of the other afternoon/early evening acts would put families off from buying tickets. However, cuting the family stuff would.
I know lots of people who will only go to a festivals (one day events) if they like the headliner and really don't give a toss who is on the undercard. It's one of the reasons I now tend to go to festivals with just my partner as I like to enjoy the whole day and see new bands. I have now been to enough Victorious festivals to know its relatively quiet during the day but gets busier late on.
I think I could put together an impressive list of would of, could of and should have seen acts that I missed in my youth because mates (different groups) would rather sit in the pub or campsite as only interested in the last acts. Maybe its just me and its only my opinion.
Why do we spend so much time talking about headliners if they are not the main draw for the average punter? By the way, I dont class efestival members as "average punters" I consider them serious music fans and hence why we care about the other acts.