I still change my water pretty frequently, like once every couple of weeks, even in the NPT's? It's more to clean the plant gunk out of the gravel than really to affect water quality.
About the NPT's being more prone to disease...I have found just the opposite. The increase in water quality from the plants and the natural feeling to the tank seem to make my guys healthier, and *less stressed*, which, to me, is the key to keeping bettas healthy. :)
It's been interesting reading on this thread though, thanks for the links!!
I had a guy who had lymphocytosis for about a year before he died of, I believe, old age. He was about two and a half...which for a petstore betta is a pretty good run.
I found some pretty horrible "cures" for it...dosing with copper and malachite green...geeze, the cure alone was going to kill him. :P I just kept his tank clean, and when he had a spot that opened up, I would put some general sulfa-based antibiotics into his water for a couple of days to prevent secondary infections, and he seemed to be all right. The lumps looked pretty awful, but he was happy, ate well, active, so I figured he was happy in himself. :)