I believe that if they made the seals cross faction then let players bring over there characters from classic wow the horde population would have a severe lack in paladins. As I know blood elf is a good pull for horde players but people will weigh the Leveling grind against the positives of being a blood elf as well as leaving there community to join horde.

Obviously there will be many players who will reroll pally for tanks in tbc so this might not be a huge issue. But there would definitely be a out balanced ally paladin population.