-snip-
Probably to save the battery for when it's needed.
Having it hooked to power all the time reduces its life, since there's still some discharge/recharge going on.
That's a myth.
No its not, doing this damages batteries, sure you can keep going 99%-100%-99%-100% but its Going to hurt your batteries lifespan.
You're probobly thinking about how a car battery works, They sit at nearly 60% charge average, wich is much differant than 100%
Goahead, charge a fully charged battery, tap a bit of current out of it, Fully charge it again- Repeat, That battery will die faster than the battery that goes from 1% back upto 100%
0%-100% can also hurt a battery