You know what here is a fact I have a Pro-96,97 and a GRE PSR-500 and about 12 more old handheld scanners with in view most are very old and they all have one thing in common they and I MEAN EVERYONE of them has been charged with the battery's installed and charged with a AC charger end of story and they still work.
You have had good luck, but many of us have not. I have had the current limiting resistor in a BC245 burn up - literally.
All sorts of things can happen - voltage surges, other components shorting out, etc.
It's just a lot safer and easier, in my opinion, to use an external charger. Actually, I always have 5 sets of NiMH batteries with me. A set in my scanner and camera, a spare set in my pocket and 2 spare sets in my camera bag.
Also I no longer use standard NiMHs because of their self discharge characteristic. The new "hybrid" type that hold their charge for months are great, and you can get them at Walmart for the same price as the standard ones.