As you say, best consult the manual.
I live with the fact that the battery will die. My current cycle count is 236 and charge capacity is below half the design capacity (battery info report).
It is handy that the machine stays up when the power goes down, we lose power out here in the sticks fairly regularly. The elapsed time life thing is news to me, I'm off to browse the internet to look into that one, and perhaps revisit my opinions if its true.
The batteries do naturally lose charge, so if you are permanently on the mains, I would expect there to be a small amount of recharging going on from time to time.
I happened to be looking on the internet for a new battery the other day, batteries from china seem amazingly cheap. I bought a li-Ion replacement for an ipod for 1.99 + 3.50 postage, it really does seem to work as described, and very cheap. At the price, (£35) perhaps I would not care as much as when I was looking at £145 from IBM.
I would just worry that they might have less than perfect quality control and start exploding....
BW