I know of one that failed because of this.
The turbo had been rebuilt by Kyp from ATP a year before it failed.
The turbo failed because the owner was constantly coming on and off the throttle (doing burnouts) on a particular occassion.
It was running 16psi of boost when it let go.
Kyp rebuilt the turbo again and put it down to the constant on / off throttle application (doing burnouts) and the amount of boost it ran that caused it to fail.
Had it been running 7psi it would not of happened. This is because the turbo is spinning a lot less at this boost level and is therefore 'de-spooling' a lot less when the throttle is shut and putting a lot less stress on the shaft and bearings than it would have running 16psi and above.
Given the above, I also conclude that BOV or recirculating valves were put there MAINLY and INITIALLY for emission reasons and NOT for turbo longevity reasons.
When Nissan introduced the BOV in its vehicles they weren't running high amounts of boost pressure. Turbo longevity was not much of an issue for them. What was an issue was meeting and passing emissions. I agree with Simon's rationale on the first page. Then as Nissan began running more boost through their turbochargers in later years a nice by product of the inclusion of the BOV was that it was also helping the turbo survive longer from those people who would drive them hard/abuse them (ie come on and off the throttle constantly) in an effort to reduce any warranty claims.
As an aside, in one of the HPI dvd's, I remember one the owners of a large japanese tuning workshop saying with regards to their GTR drag car that they experience turbo failure often because they run a high amount of boost pressure and no BOV's.....food for thought.