You may not be able to afford a £30k car but you must still have a reasonable amount of disposable income to lease a high performance car eg BMW M5 is still going to cost you circa £500.00 pcm. If you have that money burning a hole in your pocket its a great way to drive round in a £70-80k car.
Plus, if you have it on a 2 year lease you dont have to worry about buying tyres for it or MOT'ing just give it back.
I dont think this argument is black and white - you have to accept that there are shades of grey - ie everyones personal circumstances, and indeed personal preferences mean that leasing for some will make sense and for others it wont.
Look at the lease deals on a new Skoda Octavia vrs for example - if you want the tdi saloon version can be had for less than £200.00 per month -- now at that price how on earth does it make it more sensible to buy it? It doesn't unless you think you will love the car so much that you will want to own it for 8-10 years, like some people on here will end up doing with their Mk5 Golfs