Keeping the battery in the tool should be fine. They will run down on their own over time and should be recharged according to what the manual says. Leaving them continuously on the charger is not a good idea. Rechageable tools are consumable...in other words, don't throw your corded tools away. Someday a few years from now you will want to break out the ole 120 volt drill when your cordless one goes caput right when you need it. Then you will price new batteries, have a heart attack from sticker shock, and die.....or at least you might attempt to throw your drill over the house.

Most chargers today are "smart" meaning they charge the battery, then maintain it. And most good tools come with a spare battery, leave one in the tool and one in the charger. A smart charger does not overcharge a battery that is left in it. In my experiance they are no more "disposable" than a corded unit. I have 6 cordless Makitas drills and 2 Makita right angle drills and a couple dewalts (at work). and they last as long as the corded units of the past. Most people will argue they have a 30 year old corded and will never get that from a cordless but those people don't use the tools alot. I still only get 5 to 6 years from a drill cordless or not. Normally some Idiot drops them off a building,leaves them at the site, forgets them in the rain, , destroys the chuck, steals it, chops off the cord. etc.

I have even had a "smart" charger ruin a battery pack. I guess it didnt help that I was charging in sub-freezing conditions. Thats another thing, drill batteries really don't like to freeze. I've been through many cordless tools and I now realize that corded is the best bet. Cordless is great for some stuff bot for anything extensive and long-term use go corded.

For an everyday user the cordless may last as long. No debate about the quality of the motors or chuck. Batteries can and do sometimes go bad on smart chargers. The fact is that after several years the batteries will be used up and need replacement and the expense is greater than the tool is worth. I still use cordless tools and they are getting better but batteries are still going up.

I anticipated battery problems when I was in the market
for a cordless, so I bought a 12 volt drill about 5-6
years ago. The drill came with 2 batteries and recently
one wouldn't hold a charge. So I disassembled the batt.
pack, (it is full of ni-cad 'C' cells), and used the
shell and some wire and rigged it up to run off a lawn
mower battery. There are some 12volt gel-cell batts that
are available and I might try one of those, as they are
small enough that you could fashion a way to hook it to
your belt. By the way, the 12V drill has plenty of
power for my household use.