Dont tend to go very far without the little fella in tow!
Only work a couple of evenings a week, so will use Jimbobs car!
Would be more helpful if they concentrated on untaxed and uninsured drivers instead of frightening people! 
You can do some things without the disabled person in the car, if it is on their behalf, and this does include going to the shops or to metings on their behalf or in their interest etc. So It ives a fairly wide scope but is still abused by some. There again so are disabled parking places, a really personal annoyance.
I was just reading the thread , but Mike has picked up on the same thing.
Most of it is common sence.
Things like parking in disabled parking area's (with out the disabled person being there)
A disabled persons does not have to be in the car, but the basic premis of being granted a Disabled tax disc is to help that person, so shoping on thier behalf is feesable,
So is paying thier bills and booking thier holidays in town, the fact you are going on holiday as well is a bonus, and if you pay your bills at the same time etc.
I cant exactly see a police officer or traffic warden complaining about you shopping unless you are buying a pink dress and the tax disk is in a mans name. but then they could have paid for that pink dress as a birthday pressent for you, unless of course you are a man, even then the case could be argued.