Brad's right - variety is key. Truth is that mealworms are about the worst commercially available feeder as far as nutrients, meat/shell ratio, and pretty much everything.
But - they are cheap as heck, easy to keep, easier to breed than anything, and to put it simply, they're different.
I don't feed them often, but I do use them. They have a different nutritional content and taste than superworms (they have to - they just have to...).
The added crunch they provide is not to be ignored. Chameleons will have problems with too much or too little chitin in their diets. Mealworms are a good source of that.
And my favorite reason to use mealworms: beetles. Superworm beetles have a strong chemical defense that most chameleons will not tolerate (unless you wash their rear ends off first). Mealworm beetles are usually eaten readily, and, while not so healthy in and of themselves, provide great variety. Chameleons love beetles and shiny, crunchy things. It's hard to give that to them without using wild insects. Mealworms provide that.
My CB melleri loved the things growing up, and would prefer them to anything (even flies at some times).
Mealworms (Tenebrio molitar) (excuse my spelling, I'm not feeling good and am too lazy to look it up)are normally very small - maybe an inch and a half, with orange color on the tails. They have very small legs, and can be kept alive in a refrigerator. They pupate when they are ready. They are a common pest species across the world. They are fine is just thrown in with some sort of grain.
Zoophobus morio(superworms, super mealworms, king mealworms, zoophobe's, morio worms, etc.) area south american beetle larva, much bigger than mealworms (3") and have dark, often black tips on their tails, they move much faster on longer legs, and can kinda bite you (those always get fed off first... I know, petty insect-vengence, but it helps). They require more moisture, and will die if kept too cold. They won't pupate unless they get really big and old (takes MONTHS), or are separated from food or one another (bascially food as well, they're cannibals).
Confusion: some mealworms are fed a hormone that inhibits pupation. They grow really big, but not as big as superworms. These are USUALLY called giant mealworms.
If you see the two side by side, the difference is clearer.
here, I found a good picture on google:
http://z.hubpages.com/u/281935_f260.jpg