Ok, I have to disagree here. There are two main ways to use Dragon Disciple.Dragon Disciple is already a strong class with a D12 and numerous stat boosts (if you're keeping core) and although it may seem reasonable to swap out the stat boosts for spellcasting progression, bear in mind that spellcasting, even sorcerer spellcasting, is probably the strongest class feature in the game by a large margin. Even 1/2 spellcasting on a d12 prestige class is pretty strong.
1) A pure warrior type, with one level of Sorcerer for wand usage and to get those bonus spells. For this purpose, it's a not very powerful but ok class with some fun flavor. In this case, getting extra spellcasting would be pretty minor because you're still mostly in "wand usage or party tricks" territory.
2) As an actual gish (aka warrior/mage split). For this purpose, it's terrible, trashing your casting completely for buffs that don't at all compensate. With 1/2 casting - it's still actually rather weak. But less so, I guess, apparently enough less so that it can be the MVP in a very low-op game.
HD are kind of a red herring, IMO. While a d12 HD does have some utility, the actual amount gained is like 12-18 hp. That's not nothing, but it's going to be less of an impact on overall survivability than AC, saves, or defensive spells/abilities.