Frame longevity is also a function of how it is stored and treated. A steel frame ridden mostly in dry weather, not used near salt water and kept in temperate storage will indeed last for decades. A friend has a 1971 Raleigh Super Course built from Reynolds 531 tubing that has never been internally treated and is still structurally sound after 41 years so Frame Save or it's equivalent certainly isn't essential for most bikes. However, if you live in an agressively corrosive area, ride the bike in inclement weather and/or store it outside a lot, Frame Saver is cheap insurance.

I treated my new Surly Cross Check frame and fork with HDMP when I first got it as it is my rain/snow/beater bike and is ridden almost exclusively in lousy weaher. Would it have lasted a long time without the treatment? Probably, but now I don't have any worries and for the $2 or so in materials why not?