ok, this prompted an experiment from me
given that I didn't know much about batteries at all, I looked some things up. so Lithium Ion batteries don't have battery memory like Ni-Cad, but I downloaded a program called BattCursor, which gave me some information I didn't know existed
my battery is rated 40,604 mW. I bought it about 4 months ago (so basically it's new)
over the course of about 10 days, constantly plugged in, I got these readings, per day
40571
40448
40337
40315
40260
40215
40160
40093
40038
39.993
so lets say that's losing on average, about 50mW per day
that's losing about 0.1% of the battery capacity, every single day
so we'd be talking a knackered (stuffed/bodged/name of choice) battery after about 3 years, if constantly plugged in, and assuming the degradation continues at 50mW per day. which it may not.
once it got to under 40,000 mW, I chickened out and decided to try unplugging it - for all I knew discharging to 40% might lessen the total rated charge. on the first unplug, BattCurser capacity went straight back to 40604 - so it recovered from a minor dip of the 40604.
this battery was an ebay "similar" model, my original genuine Fujitsu battery that windows was telling me to "consider replacing", had been constantly plugged into the mains with almost no removal, for probably 5 years.
Battcursor told me the now-duff battery had a capacity of about 12,000 mW. at about 90% displayed "remaining" status, my laptop died - so windows was out by 90% reading 12,000 mW, so after being constantly plugged in for 5 or so years, we could say the battery had an *actual* capacity of about 1,500 mW? or 10 minutes
so this "auto switch off at 98%, auto switch on at 40%" could really be a useful utility/capability.
what I've actually done? I bought an inline switch off ebay, chopped my laptops 12v adapter cable, and I've put the switch inline, so once or twice per week, I flip the switch for ~60 mins, and let it get to 40% or so. (this means that I wont eventually wear out the kettle lead socket from physically unplugging it once or twice a week)