Similarly, you want to argue that you can continue to advance in a PrC for which you no longer meet the prereqs and / or retain all the benefits of that PrC. Sometimes, this will not make sense (i.e., a PrC that grants you bonuses when riding, but requires a certain number of ranks in the Ride skill; you lose those ranks, and no longer know how to ride at all; why do you still know how to perform particularly impressive cavalry charges?). So, should rules really be made on the basis that they sometimes make sense?