Really I tried to keep this simple but here we are in a pissing match.
What you are failing to mention on Li Ion powered devices is that most smart devices take into consideration that a Li Ion should not have a full discharge and their charge circuit will shut them down above the point the battery will be damaged.
There is a lot of contraivercy on LiIon. If you read the links above that were posted if they are worth a crap they will talk about damage from heat. The Apple one doesn't have it at all. Course if you need a battery in 1.5 years instead of 3 that's just more money in their pocket so why would they.
Here is where the controversy comes in.
Batteries heat when charging.
If you look you should find charts showing that the amount of heat the battery is generating for a 30-70% charge is pretty flat but once you hit about 70% the internal temp of the battery starts raising because the internal resistance start raising as you approach a full charge. It starts to heat at an exponential rate and has to slow the charging to prevent overheating.
From one of the above links.
The speed by which lithium-ion ages is governed by temperature and state-of-charge.
It glosses over the fact that they create heat when charging. Anybody that has ever picked up their phone as it is charging will notice the battery is warm sometimes hot. It is being warmed from the inside out. This is detrimental to the battery as well.
Time wise the time it takes to charge a battery from 20-80% is about the same time it takes to charge from 80-100% because the internal resistance increases so much as the battery comes up to full capacity.
When LiIon came out there were all these instances of products melting or catching fire from crappie desgned charger set ups. Union carbide even managed to burn down their factory in India screwing up with Li based batteries they were producing and some reports say they killed 8,000 (no typo) in the process.
http://epoch-archive.com/a1/en/us/ny...0091204_NY.pdf
Back to the point or the argument that this has become: Why subject that battery to that heat daily when it has the capacity to go two days before it needs it?
Yes there is a thermo protection device in the battery that will shut down charging if the temp gets too high but still if you think of the logic "topping off" when that 70-100% part of the charge cycle is where the battery runs the hottest. So is the 40-100% charge habit every 2-3 days less detrimental or is the charge 75-100% every niight? Thats the argument.
Lets bring it back up when somebody fry's their iPhone battery. I continue to wait till I get down around 30-40% of charge before recharging as Motorola, Nokia and the rest of the manufactures that I have had repair classes from advise to do.