Andy, I just registered and came across your question.
I also used the backup battery just once – it died in 3 weeks time.
On second thoughts I dropped the idea of the backup battery after weighing in:
– you do not lose memory settings, defaults, etc – they ARE stored provided you save them in SETUP mode
– all you lose is the clock, which I tune every time I start my work with MTR5b, and it is not so much hassle for me, as opposed to the hassle with exchanging the ebattery (open the case, etc) not to mention finding the one with a reasonable price – it is not so common battery model for the area I live in.
All in all, losing just clock does not bother me, and I am happy without the backup battery