We do it for £300 on the 1.9 model. They will most likely eventually fail - as the buildup of carbon/paste inside the manifold gets worse, the flap movement is restricted while the spindle is still being driven by the motor, this stresses the spot welds and they eventually fail, the engine does the rest and sucks the flaps in.
Good overall engine health & driving style can mean the difference between slow or rapid build up of the sticky black death carbon/paste.
As pud says, the build up of gunk will make it work harder and put stresses on it that can lead to it breaking. This is mainly due to the EGR, get it blanked (Autolusso do an ECU map and blank for £160) to prevent any more gunk in the manifold, early enough and it won't become an issue ( and make the car more responsive!).
You can also clean out the manifold (a big job best tackled at a belt change), or spray EGR cleaner etc in the manifold which helps to loosen the gunk (and a lot of black smoke out the exhaust as it gets burnt), less expensive ( and effective) than a full clean it can still help.
Have a look at your map sensor, it'll gives good idea of the condition of the rest of your manifold.
I doubt blanking the EGR is enough. A careful look at my failed flaps shows the underlying problem to be the very small plastic bearings that locate the top of the flap spindle. The bearing is barely 2mm ID, and what happens is that over time it elongates sideways. This is going to happen due to vectored manifold pressure variations hammering the flaps hundreds of time a second, when partly open. The spindle is then free to displace sideways, jamming the flap into the side of the port and eventually fatiguing welds until they break.
So build-up of oil/carbon gum isn't the main problem. It's soft and gooey enough to get pushed out of the way. My flaps and inlet tracts showed this happening. It was metal flap to metal port contact that was the issue.
Having an abrasive mix of carbon and oil will shorten the plastic bearing life, but I am pretty sure it will fail anyway due to being so pitifully underengineered. If so a blanked EGR is at best going to delay failure some.
The good news is that once the flaps are removed and EGR blanked and mapped out, a whole heap of potential and recurrent problems are fixed for good. It is *the* mod that should be done to every 1.9 CF4 as soon as you get it, as important as cam belt service but you only need to do this once.
PS I had 2 flaps break off - 1 at 75,000, 1 at 78,000 mls - causing head and valve damage. Ned at AL said he's seen it as low as 30k mls. I'd guess it's about the same level of risk as a cam belt, anything past about 50k is gambling against odds that get steadily worse. There's probably a bell-curve distribution, with a peak around 90-100k and a few cars making it to 120 or 130k, from reports on these forums.