honestly I don't think its that big of a deal and here is why.
1. There is a spell that duplicates the abilities of the class for a decently long period of time, and actually does it better than 10 levels in a prestige class does it (like become collossal and exceed the HD limitations).
2. Considering all they have given up 10 spellcasting levels which would have been close to enough, or enough to have the shapechange spell listed above, plus a whole load of other abilities or spells.
3. They could just shift into something that had fast healing anyway as that is a ex ability.
4. It is a move equivalent action to change still and they would not gain back all their HP, only equal to their level. So if they want to change form every round to gain HP they give up their move.
Considering all that the shifter gives up and how his own ability can be overshadowed with a single spell. And considering at that level of play there are alot of dangers that don't do damage, and there are many creatures that can do far more than 20 damage in a round I would say its really not that big of a deal.