Whether or not you believe in the effects of radon I can tell you this. The real estate market (agents, inspectors, and therefore buyers) have all grabbed onto the radon notion and are not letting go. That means if you're trying to sell a home and it's got above a 4.0 level, you're going to be asked to mitigate it. You can tell them that is bullshit all you want, but it won't matter to a buyer, they will want it done.
So Bob, if you're buying, it's better that they already paid for the system. Otherwise you'll just delay paying for one when you try to sell.