If I could speculate, I'd say it was because they set Dart up like that so that Burning Rush would be his first SP-heavy addition, as you'll notice most characters alternate between high SP and lower damage additions and the reverse. I'd say Burning Rush was intended to be his second addition, but they realised that it made no sense to give him that addition before he'd actually awakened his dragoon stone.
So they hurriedly swapped it with Volcano because before you get SP there's no point having a high-SP addition.
It's that sort of reason that I don't use Shana (after getting Rose), Haschel, Meru, or Kongol before getting their spirits, though with Kongol it's a short walk right after getting him. I use Dart and Lavitz because I have no choice. It would've made more sense to swap the animations and hit counts, though in effect it would be weird since Volcano is essentially a beat down.
But it's a good idea to use your available Additions 80+ times, so that way you'll gain more SP/Add once the D. Spirits are in your possession. First time you use Additions raw, you gain such little SP.
Please visit my FAQ and forum at http://www.gamefaqs.com/console/psx/file/197765/55793 and http://z7.invisionfree.com/Darkaqua/index.php?act=idx
Actually in Haschel's case, if you plan to use him as a main character, you should use him right away even before his DS. Two reasons:: you will always have a backlog of additions on haschel until omnisweep, and if you can work on flurry of styx, it gives almost no SP. So if you can level up and start using that in a way you will do yourself a bit of a favor SP-wise.and omnisweep-wise.