Correct, <3.6V is advance notice, <3.4v expect it to die soon.
I think I choose them because of ease of maintenance. I have an apple charger which you just plug these into. A Lipo (my reading) needs a special charger circuit, and I have read numerous report of them being affect by low temperatures. Sydney does drop down to a few degrees above zero at night in winter months, so I saw this as a more reliable option.
Getting 1-2 years off a single charge is perfectly adequate solution, and could very easily extended by reducing the frequency of sampling and transmitting data.
It would be a pretty simple software mod to know, exactly, what you're battery voltage is. If you compensate with the VDD reading, then, as long as VDD is 3.3V (ie, the batteries are supplying enough voltage for the regulator to be operating within its control range) then your direct reading will be accurate, but once your VDD reading starts dropping off, then the scaled reading will still give you an accurate reflection of battery state.
Having said that, though, these batteries knee pretty sharply at 1.2V so, as you say, once you're down to 3.4V, you're toast!
I agree with you about the scariness of LiPo charging. If you don't have a temperature compensated protector circuit, you're asking for trouble!
I was looking at 18650s and, while mechanically more robust than LiPos, they don't normally come with the protection circuit. I've found a few breakouts that look interesting, but, it's tough to beat a couple of lithium primary cells for remote Motes... As long as you can live with dialing back the processor speed to 8MHz (which virtually all of my Motes can tolerate).
The only place I've found a use for rechargeables is the battery backup for my gateway. Using LiPos there, but with protection circuit and constant monitoring of battery voltage, with load, even while on mains.
Tom