ECL-related mechanics end up in that situation a bit often for my tastes, like cohorts can't catch up after buying off LA since their new, lower level means they get less XP. You could, however, get a cohort with his LA already bought off and XP caught up. So it ends up in this situation where the cost is either overly punishing or completely free, which I don't think is a good idea.
Personally, I hate bloodlines because they basically add nothing. Their abilities are pretty much total rubbish, even if you get 20 levels "worth" of abilities for 3 actual levels. Sorry, WotC, but "+2 to diplomacy with Silver Dragons" is never a worthwhile ability. I think it could be salvageable if they had actually worthwhile abilities and maybe added some kind of hit die when you took a level in it too.