Prius B mode is a substitute for a traditional transmission lower gear, which would use regular engine compression to slow the vehicle.

B mode is not unsafe for the vehicle and is nothing like using engine compression for braking, as in a conventional vehicle, because the engine becomes nothing more than an air compressor.

In large road going semis, a Jacob's brake is an engine compression brake which suspends fuel to banks of cylinders, usually in pairs of 2, and uses the natural compression of the engine to help control speed and ease braking. It is very effective and it not damaging at all to the engine. Drivers of these vehicles uses these brakes on some very long downhill stretches for miles and miles.

In much the same way, the Prius suspends spark and fuel when in B mode and the ICE becomes nothing more than a sophisticated air compressor.

Please use B mode as you see fit with no concern over ICE longevity or wear and tear. B mode is there for you to use at your discretion.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)