I'm pretty sure that IS a myth...I leave my laptops (2 of them) plugged in nearly 24/7, only use battery when I take the PC out to the kitchen while I'm eating or if I use it on a trip or something. Most of the time it's on my desk, plugged in, and powered up (running folding@home).
Maybe on older (NiCd or NiMH) battery types this was the case. Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH) batteries suffer from capacity loss if they are recharged before they fully deplete their charge. If you unplugged your NiCd or NiMH laptop and used it for like 5 minutes and then plugged it back in, this could damage the battery.
Modern laptops, however, use Lithium Ion batteries (Li-Ion). These batteries don't suffer from the "memory effect" that NiCd and NiMH batteries did. You can unplug a Li-Ion laptop and run it for a few minutes and plug it back in, the battery will charge back to 100%, and the battery won't be damaged.
Also realize that most of the time your laptop is plugged in, the battery is not charging. The laptop charges the battery to 100% and then stops charging it. Most of the time, the battery is just sitting there, just as it would if the laptop were turned off and unplugged. Therefore, the only thing that could possibly hurt the battery if you leave your laptop plugged in and turned on is the CPU/GPU heat, and if you keep your laptop cool (use a cooling pad or something if you do intense processing on it), the battery won't even get warm.