Because the IMS bearing can fail catastrophically there are folks who are just not comfortable driving their 996 without some assurance that they can get advance warning of imminent failure. If you are one of them then the Flat6 guardian is one way to achieve peace of mind.
As stated above if it does light you will be spending an additional ~$1000 -$3000, depending on what else you do "while you are in there". However it won't be ~$!5-20K for an engine replacement.
Another option would be replacing the IMSB preemptively. There are at least 4 IMSB kits being offered. Pelican, Pelican uses a stock steel type bearing, LN and Casper Labs offer ceramic composite bearings that should be a more robust bearing, and there is the Engine Lifeguard out of Europe
The ultimate IMS UPGRADE = ENGINE LIFEGUARD - Rennlist Discussion Forums.
All of the above options are still bearings that could fail. So even with a replacement bearing a Guardian may be worthwhile to some folks.
Data point:
I have a 2003 996 that does not have a "Guardian" installed. I do have a LN Engineering kit along with other bits (clutch, DM flywheel, RMS, AOS etc.) ready to be installed when my clutch needs replacement.