I guess with older NiMH batteries you have to power cycle it to sorta get rid of the "memory effect" but Li-Ion batteries don't have the same memory effect problem however, they have something called "virtual memory effect".

Starting with a battery's manufacture at the factory, lithium ions oxidize in the battery, shortening its life. After a certain amount of time, the battery exhibits slightly different electronic properties, which the device can only detect when the battery is discharged.

This type of problem is known as a "virtual memory effect" and occurs when your battery meter can show a high charge, then suddenly drop to low charge when the battery is almost dead. Every thirty cumulative cycles, you should discharge lithium ion batteries until the phone gives a low battery warning, then recharge it. If you discharge lithium ion batteries to zero voltage, the internal safety circuit may open. In that case, a normal charger will be inadequate. Reconditioning ensures that the battery meter on a lithium ion cellphone is accurate (although it will not affect actual battery capacity).

Of course if the internal safety circuit fails then when you power cycle your battery thats probably why it ends up killing your cell phone battery (Li-Ion). Li-Ion battery's worst enemy is heat. When you plug your phone in to fully charge the battery after it's completely drained of power it usually pushes a lot more voltage across the cables to charge the battery and that type of heat usually shortens the life of a Li-Ion battery. So thats probably why you don't want to fully drain your battery in your phone; just when you get a chance charge it up or throw it on a charging dock. This is the only logical reasoning that I can come up with on why you shouldn't drain your phone battery. Other than that, I'm completely out of ideas.