[EVO 4G General] sidepart:
This is a good question to ask. Fully charged is 4.2V, yes. Without this kernel however, the charger will give you a "fully charged" state somewhere around 4.0-4.1V (normal practice is to set "full charge" as 10% of the capability of the Li-Ion cell). It'll charge up to 4.2V and then discharge until it reaches a top-off voltage. That's the general practice, I'm not sure what that voltage is with our particular devices. Why is this done though? Because keeping your battery maxed out at the actual full charge of the cell reduces the life cycle of the battery. At some point it won't be able to keep this charge for the same amount of time.

Take for example a laptop battery, you generally notice that after 3 years or so, they only hold an hour of life whereas before maybe they held 2 hours. This is because the life cycle of the battery has degraded. Take note that the manufacturer has put into practice the same thing here that HTC has, they're only charging up to 10% of the actual full charge. Had they not been, you might actually see your battery go from 2 hours to 1 hour in the course of 1 year instead. I don't know exactly how much it reduces the life cycle mind you, I'm just making an example.

Charging to 10% of the max voltage is standard practice because it promotes a longer life cycle for the battery. Not using a trickle charge method is also standard practice because it's hard to prove that there won't be any catastrophic issues (no matter how rare or improbable they may be, like in the case of this kernel). Either way the manufacturer is going to choose not to trickle charge primarily to extend the life of their product, with the added bonus that they're also promoting a charging method that's almost intrinsically safe compared to trickle charging.

To answer the bit about charging to 100% and then taking the battery out. I assume you mean, charging the battery to 4.2V on the stock kernel (assuming you're monitoring it) and then disconnecting the battery for a few days and leaving it like that. I don't see any issues with this other than a reduction in the life cycle of the battery. Think about this though, you're most likely not going to store it for an extended period of time fully charged, and next time you charge it you're probably not going to sit with it until it reaches 4.2V. As far as safety goes, you will absolutely not go above 4.2V if you take it out and store it. Safety concerns (that I'm aware of) come from the idea of keeping a battery at 4.2V for extended periods of time while a charger is hooked up to it. There's a greater chance that just a little spike or disruption could cause the battery to charge to a dangerous voltage. Without trickle charging, the charger is completely SHUT OFF. Because of this some random little bug or happening wouldn't trigger the battery to charge above 4.2.

Interesting side note, you absolutely don't want your battery to hit the lower voltage limit either. I think that's something like 2.5V if I remember correctly. Generally your device will shut off and say that you have 0% charge around 2.8-3V. This is so that you absolutely never hit that 2.5V wall. After that, your battery will avalanche and you'll never be able to get it to charge up again anymore. Would be interesting/hilarious to see if someone promotes a mod that adjusts the shut off voltage of the kernel! I can see it now, they'd tout it as having NONE of the potential hazards of tickle charging. :D

Also again to anyone that reads my garbage. I only have a degree in EE and worked with Li-Ion batteries used in a solar vehicle during college. I don't have any professional experience with them beyond this, so take what you will and correct me if you know something I don't. I use the SBC kernel because I'm interested in seeing an application of trickle charging on a Li-Ion battery.