ShotOne.
Duncan D's reference to low flying is most certainly not a red herring. I'm sure the SOP for one particular (ex)user of the low flying system over the sea would be to set the baro alt to match the rad alt in S&L before using the baro as the master! Only way to do it at 100ft. Had a go myself in the Bucaneer sim long ago at Lossiemouth.
Not sure about the Nimrod, but I've used a similar technique when conducting low level trials (manoeuvring) over the sea. The rad alt is great for S&L but not much else!
BB
As not all airfields are dead level flat and some can have marked differences in elevation then that "zero feet" indication might only be related to one very specific place
Agreed about airfields not being level, but QFE is invariably factored for the in-use touchdown zone. Not unusual at some fields to get a QFE change whenever the runway changed. Furthermore, most altimeters are accurate enough to read a pretty constant '0' at the threshold.
Don't however think we are using the altimeter 'approaching zero' to initiate the flare! It simply means that every airfield I fly visual circuits at I will be looking to fly downwind at 1000ft, halfway round finals at 650ft, roll wings level at 300ft over my 1 mile point. I may be a tad high or low, but it will there or thereabouts.
If you've not used QFE much then, like anything you are unfamiliar with, you'll be reluctant to change away from QNH. Trust me, having been using altimeters for over 30years in a wide a range of the aviation spectrum roles, QFE most certainly has it's place.