I'd prefer not to give my dog unnecessary chemicals too. But I was under the impression that adult dogs should be wormed every three months or so?
There's a lot of people in my area who don't pick up after their dogs (disgusting!

) and if they don't worm them either, I'd imagine the soil is going to be contaminated with eggs. Easy for a dog to ingest after running/digging then coming inside and licking paws.
Also, there are young children in the family and worms, if transmitted to humans, can cause blindness in children. Maybe I'm being overly cautious but I view regular worming as 'prevention better than cure'.