I was taught to say this years ago during a stint at Coventry (DX Wombat - you should think yourself lucky Brum give you the time of day, they used to palm stuff off on us at Cov just because they couldn't be arsed talking to it).
I still say it and the main reason why, is that my current unit sees a high proportion of GA traffic which is not based in the Bristol area and therefore is not familiar with the airspace. I have lost count of the times GA traffic has called, stated that they will remain OCAS or give a routing that should keep them out of CAS and then find myself saying 'turn right/left heading/descend below xxxxfeet to leave CAS'.
Quite frankly it gets on my tits that my train of thought whilst vectoring commercial traffic inside CAS is interrupted by some numpty who is incapable of following a simple instruction/request. It's not that easy to start avoiding action inside a Control Zone when the sequence you have set up has limited space to manouevre in.
If the standard of R/T displayed by GA pilots is anything to go by, I as an ATCO find it hard to have confidence in their flying abilities.
That said, there are some very good GA pilots out there who know their stuff and don't cause problems. If the CAA paid more attention to reports filed by ATCOs (and grew a pair of balls), they could weed out the numpties and start refusing PPL renewals based on bad/dangerous performance.
As an aside, I once filed an MOR on a CPL/ATPL pilot (who was flying a Cub on his day off) because he thought it would be funny to buzz a BA46 which happened to be lined up on the runway. He flew over it at about 100 feet despite being told not to do so and then flew down the runway at less than 50 feet before leaving CAS without speaking again. I gave the CAA his name, occupation, company, position and based airport. Their reponse, six months later they said they couldn't find him and that no action would be taken.
Covering my arse by saying 'remain outside CAS'? You bet I am!