All our family used to undercoat all the cars, used or new, when they were purchased.  We usually bought cars at 20 to 30 thousand miles, often what our dealer called 
factory cars. 

 Main reason  for undercoat in the past was all the roads we had were gravel until we got to the main highway.  Probably about 60% of our miles were put on gravel roads back then.  The undercoat not only prevented against salt, but also made a quieter ride when on gravel, as you couldn't hear the rocks hit the bottom of the car as loudly.  Downside of undercoat was when you worked on something under the car, sometimes it got in the way. 

Now we don't bother with undercoat for two reasons.  One was mentioned in a previous post, most manufacturers warrenty their sheet metal for at least 7 years.  Another reason is we have almost all blacktop roads, we almost never have to go on grravel anymore. 

We have one car we bought new that is over ten years old, not a sign of rust, where in the past our cars would rust through in three years or so.

I still don't think it is a bad idea, but some dealers just thinly paint the coating.  Be sure you get the heavy stuff, it is applied with what looks like a grease gun it is so thick.  With the heavy stuff, you don't notice road noise as much as without it, another advantage.