I had a old type PGL III green laser, and the "D" cells leaked, by the time I knew there was leakage, the laser was about toast! I worked long and hard to remove the stuck remains of the batts, and the corrosion rendered the laser nearly a total loss... in the end, I sent it back to CNI, where they replaced it with another PGL for ~$150. -Glenn
I have had NiMHs leak and I've had them swell making it a bear to remove them. Once you've been a victim of leaking batteries (of any type) by leaving them in a forgotten device, you may find yourself being more careful.
I find the more I care about the given device, the more I take batteries out when not in use. Further, I have more devices than batteries. Leaving them in is a luxury, in some cases. I am not talking remotes or every-day-carry flashlights, but about things that may get forgotten.
Just last night, I broke out the vinegar to combat a leak. I am a strong supporter of removing batteries once your "done playing with your lasers."