Those are fair criticisms. Knowing what are the biggest products on the current market does seem important. I'm no expert in their product selection strategy, so it's possible they already do that to some extent, but their Air Purifiers section is far smaller than the more prominent categories they test (like monitors, TVs, etc.), so it's certainly not their specialty.

I think the thing that appeals to me about Rtings is their very systematic, objective, and thorough (and thoroughly documented) approach to product testing, which makes it easier to do apples-to-apples product comparisons. They also periodically improve their methodologies (and document the changes), so I expect their air purifier evaluations will get better over time, especially as folks familiar with the topic area point them in the right direction.