It's strange - I've noticed people don't seem to chase the list the same as they do in Birding - whether it's the unlikely success rate in twitching a rare, or whether it's the worry of never knowing if a rare had been put down.
In Birding, there are some birds on the list that will never occur again, but birders tend to keep them all on the radar - whereas in butterflying I glean from the forums that it's the regulars that people aim for, excluding the likes of Camberwell Beauty and Monarch - it would be the equivalent of having a birding target list of all the regular breeders and those that over winter, plus regular drift migrants etc. - Clouded Yellow being say Red-breasted Fly, and stopping there.
I guess the next level that people are in with a chance of are:
Camberwell Beauty
Monarch
Large Tortoiseshell
Long-tailed Blue
Queen of Spain
Nobody seems too concerned about chasing, when news of one of these breaks - you know me James - can't help myself - I'll be on the road.