I'm lucky (if you call blowing into that little machine until you're close to passing out - lucky). We have one at work. I work with enzymes so I have to have a yearly enzyme screening along with a PFT.

The nurse on staff also told me I could use the machine whenever I wanted to if I was feeling short of breath. Last year, I took her up on that offer the day after a really crappy mtb race and low and behold my values were down. In fact, out of the last three PFT's I have steadily gone down in function.

I'm on advair and singulair for my allergy/asthma symptoms and race bikes (mtn and cross) about nine months out of the year so I guess my asthma is in "check" as well as it's going to be.

If you don't feel any performance issues, maybe you're right. Maybe just get them every other year or so....