The benefit of Improved Precise Shot is:
Your ranged attacks ignore the AC bonus granted to targets by anything less than total cover, and the miss chance granted to targets by anything less than total concealment. Total cover and total concealment provide their normal benefits against your ranged attacks.
The benefit of Precise Shot is:
You can shoot or throw ranged weapons at an opponent engaged in melee without taking the standard –4 penalty on your attack roll.
The penalty for firing into melee is not an AC bonus, and is not specified as being due to cover or concealment, so IPS doesn't cover it.
(Also, on a meta level, if you automatically got the effects of PS from taking IPS, why would any ranger ever take PS?)