Having worked in a lube shop for a year and changing these on all types of cars, I keep up on mine no doubt. I think Subaru says change every 2 years or 25k miles (might be 50k I don't remember exactly)? I would do it once a year.
I can't speak to the validity of the claims, but there are HEPA and non-HEPA filters. I would definitely go with the HEPA ones just for that added assurance. And really if you only change it once a year the price difference is totally justified in my mind.
I didn't look through all the pages in this post, but I've seen dead animals (usually of the rodent variety, but also some avian types) and feces in, on or around the cabin air filter.
Working on cars for a living equates to seeing some crazy and our disgusting stuff!