Motability cars are extremely good value for money if you choose carefully. A new car, fully maintained, insured, taxed and a with guaranteed replacement vehicle when yours is off the road is good value at £45 a week.
Yes, you can run an older car for less than £45 a week but if you're spending 4k on the car, £300 a year on insurance and say £200 a year to cover maintenance/tyres/MOT, your total outlay over 3 years is £5500. If you get £2k back for the car you've spent £3500 or near as damn it £100 a month. The £4k in the bank will earn you £20 a month in interest, so you're only an insurance claim or expensive repair away each year from being out of pocket and stuck with an older car.
I can understand opting out if you want something a bit off the wall or an older prestige car or something, but for a bog standard hatch I'd say the benefits of the scheme outweigh the costs.
Isn't this a bit academic right now though, as the car won't go back until March 08 at the earliest?