Read a technical article on the VW 3.0l diesel yesterday. After it talked about the variable-vane turbo and piezo injectors it said "Infinitely adjustable inlet manifold swirl flaps adjust as a function of engine speed and load to improve air mixing and increase torque; they gradually open from 1250rpm and are fully open at 2750rpm."

OK, so this is a VW, but very likely ours are for the same purpose.

Bit of extra low end torque in exchange for the risk of suddenly needing a new engine one day? - Mine will still come out when the warranty expires.

 

In a BMW diesel engine the swirl flaps are either fully open, or fully closed, depending on the rpms. Over 2000rpm they're opened.

What I know they are there to decrease particle emissions on low rpms. There might be an intention to get a better air flow into combustion chamber too to enhance the fuel burning, but I haven't noticed any differencies in the engine running on low rpms or in fuel consumption after removing them.
And at least in a Finnish MOT test they don't recognize the difference between a car that has swirl flaps and a car that doesn't. They measure only smoking rates in the emisission test and moreover that's done on the highest rpms the engine runs.