If you've got vast quantities of money and are simply not bothered then there is little reason not to just order whatever you want, new, to your spec.
But this doesn't describe most car buyers. Most car buyers are borrowing money to buy a car - not in itself a problem - but if you are far from flush you have to wonder whether it's really that sensible to take out a whacking great finance package on something that'll shed an enormous pile of cash in its first year. Most people buying new do so with a pretty small deposit so end up in negative equity pretty much right away. And for what? The new car smell? I've never driven a sub 12 month old loan car that didnt still have the new car smell. Your ideal spec? Fair enough on an A6 or something but just how much spec can you add to a Fiesta or something? Plus it's just as used as everything else once you drive it away.
I was lucky enough to drive a brand new car for a few months once - it was a box fresh 320i with 6 miles on it. To be honest with you it didn't feel appreciably different to the other E90's I'd subsequently drive - all with between 1 and 5k miles on them and worth a lot less. I shudder to think how much money that 320i lost between the day I drove it away and the day I handed it back. Luckily it was not mine and just a loan car.
It's especially confusing when the choice to buy new - an extravagence - is combined with money-saving-eco-stuff like low road tax. Why would anyone care about whether the tax is £35 or £135 when they've just spend £15k+ on a brand new car? It makes no odds. The fact you care about saving money on road tax or spending a bit less on fuel in the first place whilst ignoring the far bigger saving you could make is just bizarre. People obsess about little details like tax and mpg whilst either refusing to face the depreciation thing or pretending it doesnt matter.
Nearly new cars bought through approved used schemes are the way to go in my opinion. It's still new enough to smell nice. It's still got all the backup and warranty you'd want, which is usually easily extendable for far less than you'd have lost in depreciation on a new one. Most people buy dull diesel hatchbacks anyway so there are plenty of these around in a range of specs.
The reason why I couldnt buy new isn't because i cannot afford it but because I cannot think of a situation where there isn't another nearly new car I'd rather own. Lets take my favourite car, the 5 Series. Housey just bought one for about £34k. This would have got him a brand spanking new 520d SE with a few options in his choice of colour. BUt he didnt buy that. He bought one just 1 year old from a main dealer for the same money - but because it wasn't new he ended up with a 530d M Sport instead. Surely thats better?
When I bought the 335i I could have, for the same money, had a brand spanking new 318d ES. No prizes for guessing why I didn't bother.
If you crave a certain car and can afford it new then, well, thats probably different. If you really want a Porsche Cayman R and can afford a new one then, well, there probably isn't anything nearly new you'd rather own and its not as if a Cayman R is an eco-purchase anyway so why not. Ditto the A1 example above. If the A1 is really what you want and nothing else at any price does it for you then, well, never mind, order one new. But thats not normally the case with most purchases.
But a brand new Focus 'Style' diesel with £35 road tax? What a waste of time. Just buy a 6 month old Titanium instead.
There is a different scenario where it can make sense, too, but one that few people do. If you intend to keep your car for 10 years then arguably the difference between new and used becomes insignificant when depreciation is calculated on an annualised basis over such a long period time. Some of me fancies ordering a new 5 series one day and doing exactly that, but I bet when I came to sign on the dotted line I'd bottle it and take the 3 month old next model up instead.
Because the best way to make sure you get the spec you want isn't to max your budget on the options you want - its to buy a car some other poor sap spec'd up with absolutely everything

The 'my spec' thing is heralded as a huge thing but most people dont buy a brand new car exactly to the spec they want as the budget ends up stopping them ordering exactly the options they want and they end up compromising anyway, ie the guy with the 118d who ended up not ordering Xenons etc.