I think this has been discussed a few times before and although I don't have a problem with it, especially if the later era classes are down on numbers, it could lead to a problem if too many people from earlier classes want to ride up a class.
Also, where do you draw the line? Can pre75 ride in pre78? Pre78 ride in evo? Pre65 ride in pre90 etc.
Like I said I don't have a problem if the race is down on numbers, maybe in that scenario a few bikes/riders could be invited to 'ride up' a class just to get a few more bikes out there and make it more interesting.