Law Firm Plan Design

A number of law firms seem to have 2 separate plans for Partners and Associates. I understand the goal is probably to allow the partners to receive a greater profit sharing contribution...is this the main reason? Forgive my ignorance on plan design but isn't it more efficient and less expensive to encompass all levels of the firm in one plan?

There could be another reason or reasons in any particular case but the traditional reason is to avoid having to make top heavy minimum contributions for the associates. 

I have seen firms like this (including accounting firms and doctors offices) have two plans to allow two different sets of people to work on the plans. 

In those cases the regular employee plan is worked on with HR based people that can see the employee's compensation.  The partner plan is worked on by a very small and select group of people and the outside TPA who are the only people who get to see the partner's compensation. In short the partners were willing to pay money to make sure their compensation wasn't common knowledge even within HR. 

It has been a very long time since I worked on a law firm's plans but I seem to recall the exact legal form of ownership can matter at times.