I don't know if it's been suggested (as I don't want to read the entire thread LOL) but how about assessing rarity as the average number of times it gets played:

total number of plays live/number of years since the songs release

This would surely give a truer reflection of how rare a song really is?

Even more accurate would be a total number of plays/number of shows since the songs release, although who could be arsed putting a list like that together? <cough>Seb?<cough>

That could be the defining rarity list for the Jovi catalogue!