Having flown the HUD (in which flight path vector is the primary focus in most phases of flight), I find it handy (when I don't have a HUD) for:
1. Engine out acceleration;
2. Raw data (i.e. no flight director) hand flying, such as visual circuits (both for maintaining level flight and subsequently establishing 3 degree approach, or steep turns, or various other raw data manoeuvres.
I find it more useful in simulators than the aircraft, mainly because the above manoeuvres are generally done in simulators rather than the aircraft, and there is less "feedback" and/or "feel" in a simulator than in a real aircraft, so the more aids the better. But having practiced using it in those scenarios in simulators, if I ever found myself in one of those situations in an aircraft on a dark and stormy night, I would probably use it. Hence I always have it turned on.
You can, of course, achieve everything just as easily by scanning to the IVSI, the only difference being that the FPV is closer to the centre of the ADI so can be monitored more closely (or even continuously) while flying attitude as the primary instrument.
I once believed it may be useful in an unreliable airspeed scenario, but Boeing have since updated the checklist to say it may not be reliable (as noted by previous poster). I actually really like that advice, because attitude and thrust really should be the only things you are thinking about in an unreliable airspeed scenario.
Dismissing it as a toy is a bit rich. It is not intended to replace a flight director, but in the event you don't have a flight director it becomes more useful. In fact, in a HUD, it is generally used in conjunction with a guidance cue, which is simply a flight director referenced to the flight path vector rather than the aircraft nose.