Personally, NB's are built as cheaply and quickly as possible if you go from one of the main builders.
I have recently been looking at a small local builder who does groups of 10-12 houses at a time. The build quality of those look a lot better and the standard spec is a lot better also.
NB's also seem to have a lot less physical land with them, they are squashed on a small plot and are generally overlooked with tandem parking.
That still doesn't explain to me why you'd only want to live there for 3-4 years versus not buying it in the first place. If the quality isn't up to scratch, why would you be living there? I can understand why someone might choose not to buy a NB, but if they did choose to buy one, why would they do so expecting to move out relatively quickly?