The problem isn't usually overcharging, it's trickle charging.
The way a battery charger works is that it will go into charge mode, charge up the battery and when the battery's electron belly is full, it'll say "I'm full!" and the charging system will go into trickle charge to maintain the battery at full capacity.
That's all well and good, but there's a complication because the trickle charge isn't nearly enough to power the device and keep the battery charged up too.
The Lithium Ion battery chargers tend to err on the side of caution because an overcharged/overheated LiOn battery had a tendency to explode back in the early days of the technology. Now they're vented and the chargers are smarter so that doesn't really happen much anymore.
The problem with a device like the DROID and battery life is there are so many variables that concern how much power the battery is consuming that it's hard to keep up with.
As they say, there's an app for that. Seriously. From the Apps menu, Settings->about phone->battery use.
Those are guesstimates at best. The charger couldn't keep track of all the demands on the battery and the best thing to do is to make the job less complicated by killing the apps running while the phone is charging.
The less the phone is doing, the better. Kinda like when you're eating, you don't want to be doing a whole lot else or it could get messy.