The following link is to an MAIB safety digest for 2007. Case 25 involves a RIB and discusses prop guards:
One of the conclusions was:
"This accident highlights the dangers
posed by unprotected rotating propellers.
Had a propeller guard been fitted to the
safety boat, the terrible injuries would
probably have been prevented. Such
guards can lead to reduced acceleration,
speed and manoeuvrability of the boat;
however, the benefits of a safely guarded
propeller have to be given consideration
compared to the boat’s potential loss in
performance. It is suggested that the
requirement for a propeller guard will
depend on the exact role and particular
operational conditions that a safety boat
is likely to encounter."
It is an interesting read, although the safety digest version is much abbreviated.
Personally, I use one occasionally at work if I am worried about fouling/obstructions in very shallow water, I also use one when I have kids in the water playing on boards/tubes etc, and I also recommend them on the sailing clubs dories where performance isn't such an issue. Otherwise, if I am driving the boat myself I am happy not to have one on.