I think it may well be a mixture of both. Most of the operators advertise themselves as the alternative to the train, and whilst they win on guaranteed seating and price, they lose badly when it comes to traffic and the resulting unreliability. It is therefore quite useful for operators to keep their commuters satisfied.
On the other hand, the coaches do do a whole collection of work during the day: Clarkes, I know, have a/the Globus hotel/airport transfer contract (and can often be seen parked directly after the morning peak in St Martins Le Grand), whilst Kings Ferry have schools contracts/work, do work for the Orient Express, and have a/the fog transfer contract for City Airport - as well as NatEx reliefs. Where possible they send coaches up on commuters to be more cost effective - hence why VIP coaches appear on commuter services from time to time - but sometimes they have to go 'light' simply due to timings/hours.
Chris