I don't know if this has been reported yet (didn't see anything using search function), but when you talk to Dances with Balrog a second time without collecting the 30 Dark Marbles, you will still be allowed to do your Job Advancement (2nd, that is). I forgot to take a screenshot on my Spearman earlier of it, but you can easily test it...
Should now be fixed. Can anyone confirm please? Edit (Gi): This applys to warriors, magicians, bowmen and rogues. Can any of these classes conform that it's working properly?