On NW/CO, being plat means (almost) never flying in coach domestically. Cutting through check-in and security lines is also nice, as is the extra attention that they pay to me at the airport when the $hit hits the fan, as it often does.
One perk that I am becoming more and more enamored of is the 125% mileage bonus. On transcons, and trips to Europe, the miles really pile up.
But of course, I am talking about top tier. Would it make sense to keep low/mid-level? I don't know.