I've never heard of it being a bad thing not to get sick. My almost 3.5 year old has never had anything except a low-grade fever once after a vaccination - no runny nose, cough, stomach bug, etc. (knock on wood -- I know she'll get one sometime!).
She did breastfeed until almost 3, and I'm at home with her although we do lots of playdates and classes. She goes to childcare at the gym several times per week and I thought when we started doing that she'd for sure come down with something, but not yet. I think it's just a combination of luck, environment, good food and rest. We do always wash our hands when we come in from childcare, errands, or any other activity where we're touching lots of public stuff like shopping carts -- but I'm not weird about germs either. She has an older sister in school and my dh works in a large building with many co-workers, so they do bring home bugs on a regular basis.
I've heard the rationale about kids who get sick a lot in their toddler/preschool years being healthier when they're older and in school because they got some of the more common infections over with when young, but eh. I guess I'd rather wait on whatever illnesses she'll get until she's older and better able to cope. I don't blame folks whose little ones are sick often for finding the bright side, though. I'd feel the same way.