In my view it's poor practice, largely for the reasons that you highlight. And, unfortunately, I can't give you an answer to your questions.
As a general principle though, an aircraft should be able to vacate onto any exit unless you are told otherwise. Therefore it is probably better to stay with TWR until you are on the taxiway.
BUT, it's just my view and I have no doubt others will have different views.
As for how a TWR controller controller can get a message to an aircraft that's on GND - the TWR and GND controllers will almost always be sitting very close to each other and can talk whenever something like this happens. The problem is that when it is left up to the pilot to choose when to change frequency time can easily be wasted trying to find out which frequency the aircraft is on - especially if it is in the process of changing when the controller calls.
On the whole, "vacate and call GND" may work OK somewhere quiet but at an airport where other aircraft will be landing, taking off or taxying around, and 'situations' can evolve quickly, it is too vague.
Last edited by Spitoon; 14th Dec 2012 at 04:58.