After doing some research for a GN we are building with a large and EXPENSIVE turbo, I contacted a few very experienced builders of high HP turbo applications who said they always include a BOV.
One of the reasons is just what Ted stated above - damaged throttle body blades. Much worse damages are compressor wheels, bearings and housings.
For years, the Buick community has been in denial in the use of a BOV. With a stock GN, it make no sense to use a BOV as the system of turbo, intercooler and displacement does not produce a shock wave of any significance.
But it is easily seen, or actually heard, that with simple mods, compressor surge is present. Is this going to damage parts, probably not at a moderate level.
Go to Ted's level, or even less, and damage potential greatly increases with HP and RPM. Even though the ball-bearing turbos are more "durable" than regular turbos, their wheel speed - RPM - is much higher, so the shock wave is much more intense.
Over the past few years, we have experienced more than a few damaged turbos, mostly BB's, that were relatively new, and had un-explained damage to compressor wheels? Most all the damage turbos were ported shroud versions? Even the slightest damage will cost more to repair than the price of a quality BOV.
I do want to qualify my statements here from the point of view of experience with street and race cars, and most all our issues are with race cars. Not I know some guys treat their cars like race cars on the street, so higher HP and RPM can apply there as well.
We see lots of non-Buick turbo applications at our local track and from elsewhere, and these are from 7-10 sec. cars, and never remember seeing one without a BOV.
With a good BB turbo costing $2-3000+, it make common sense to protect it using a BOV. There are many BOV's out there, but the recent "Q" series Tial is our choice.
This is not a commercial for Tial, but our good experiences with them.