They can work if there done right, for example, LNC/DRG could ride drakes easily, because of how DRG and Dragon Knights are about, and that is usually either killing or taming dragons, perhaps a possible BST/Tamer? class could ride any, as part of thier role of taming mobs, and other classes and jobs can ride small ones they can intimidate easily.

This could work well, but it has to be done right or it will fall flat on it face.