Re: Wee, free spare battery!
The issue with these batteries isn't that they degrade slowly, so you would notice them slowly getting warmer each time you use them; it is that they behave perfectly normally until about 10 seconds before they hit thermal runaway. (Thermal runaway is when the internals of the cell rise from ambient to ~150'C in 30 seconds and then from 150'C to 2-3000'C in 2 seconds)
Knowing how much people are charged for their batteries, I can understand completely what you say, and although I can't condone what you are saying, if you do choose to do it, be very aware of what might happen. Play safe; do NOT sit it on your lap, do NOT leave it charging or running unattended (although in very rare cases it can happen spontatnously even without charging or discharging), and if you choose to use it on your antique table, don't be upset about the burns!
If it goes up, you will get a LOT of CO2 which doesn't support life like oxygen, so get out to fresh air, and you will probably get a few cells go with maybe 10-40 seconds between each one, so if you choose to take it outside, be prepared for the flames coming out as each one goes.
I don't know what each of the company's policies will be, but if you order a replacement and get burnt by the old one you will have forfeited any right to claim for the damage.
DISCLAIMER: I may or may not know what I am talking about (so AC!), so if you get hurt after any of the above, it's not my fault :)