The problem is that True Strike is a spell designed to make it worthwhile to play a multiclass Wizard/Fighting Type. It can be cast in armor, and it greatly benefits melee combat (especially in conjunction with Power Attack, Spirited Charge, or Smite Evil).
If you have potions available, then the single-class Fighter can pretty easily duplicate the Fighter/Wizard's best trick at low cost.
I thought that you could only make potions of spells that can affect other beings, and that Shield and True Strike are caster-only.