Belgarath: Would you rather be shot and die quickly, or die slowly from poisoning? Having an IPS and not following it (without a really good reason) is "fatal" quickly - as you have provided the standard of conduct against which you will be judged. Not having an IPS and "winging it" means you leave it to the judge (never, ever a good idea) to determine the standard by which you will be judged and held to before he or she strings you up based on what they think would be prudent....
Speaking of which, I'm not aware of anything that "specifically" says an IPS is required EXCEPT that one must be a "prudent expert" (with the care, skill, etc.) as a fiduciary, and I can't think of a "prudent" investment manager who would do without one....