I think a spare for the 300 is overkill--but not for the full meal 750--so I will have the old one along again--it's 3rd time on the AZT
Seems reasonable to me maybe not with a brand new one but if one were already over a year old having a spare might save the day at some point.
My first Legend lasted over 2 years did the 06 tour and the 07 AZT300. The new one has the chip and the first ride was 12/20/08 so the first one must have lasted about 2.5 years banging around on the handle bars. The new one is still working after 3 years but I am not sure I would want to trust it for a big ride.