Given one day, a druid can find any type of animal (normal or dire) of any hit dice he or she desires? I don't think so. The 200xp cost (which is trivial beyond the first few levels) makes it possible to avoid what could be a lengthy, even futile search. You also don't have to re-aclimate the pet to your other traveling companions. And lastly, if this is a "raised-from-birth" type of pet then you would certainly rather advance it a HD instead of dismissing it. In most campaigns, a druid will probably level before they could raise a new pet.
Anyway, there are real in-game advantages to outwiegh the small xp cost.