Faults discovered on test drives - fix or walk away?

[Matt UK]

I see more and more threads along the lines of 'took a test drive, noticed and mentioned faults, dealer said they would fix, purchased car, faults still there, dealer tried but no change, dealer now an arse, please help me'.

I was test driving a dealer 5 series and during the drive the engine management light came on.
Don't worry said the dealer, it is nothing, you buy it for today only price and I will fix this for you before collection!
I replied, you fix it and tell me when you have. I'll pop back and we'll take it from there.
No, he replied, if I take a deposit I will fix it. No deposit, no fix.
We shook hands, agreed we'd both lost an hour of our lives we were never going to get back and haven't seen each other since.

Consumables I get - tyres, brake pads etc. But even then I would rather haggle them out of the price and get done myself.

But why on earth would you part with money on a car that needs work and simply trust it will be done? Seems madness to me.