Personally would try anything to make them not to do it:

Cons for the integrator:

- less revenues

- more implementation work due to implication with PC's HW and SW.

Cons for the customer:

- dependence by PC availability even for basic phone services

- lesser call quality (clicks can be heard when switching windows)

Try wagging the cost of IPCP licenses as deterrent against a traditional physical phone implementation taht give you more bang for the buck.