It's interesting that she briefly mentions that asthma levels may be lower in populations with parasitic worms, since I've read several sources that suggest the opposite: that parasitic worms may provoke asthma. Like this research, they've only shown correlation, but in one particular study, 198 of 201 of patients in Ontario with bronchial asthma were found to have worms, whereas all of the asthma-free controls were worm-free*.
If I had to make a very un-scientific guess, I'd guess that some worms (read: species) may have benefits that outweigh the costs, but that many are quite unpleasant and are best avoided.
*Source (and a very highly recommended book): New Guinea Tapeworms and Jewish Grandmothers: Tales of Parasites and People by Robert S. Desowitz.
The trouble with being a god is that you've got no one to pray to.
-- (Terry Pratchett, Small Gods)