When it comes to rust on steel boats, my sense is that exterior rust will make the boat look bad while interior rust will reduce its strength.
When I worked for Charlie Wittholz in the early 1980's we figured that steel boats had a useful lifespan of 20-25 years or so, depending on build quality and maintenance, before they needed sufficient replating and reframing as to be of negligable value. At that time we were calling for the interior of the boats to be sandblasted white and coated with zinc rich, coal tar epoxy. The zinc was supposed to greatly improve the adhesion of the coating and the coal tar epoxy was very hard stuff that would stand up to abrasion very well.
Over the past ten years I have been aboard a number of the steel boats that were built from plans prepared during the period that I worked for Charlie.
In several cases I had a chance to examine the interior skin of the boat. In all cases I found that there was as significant rust along the stringers, in places as much as an 1/8" of the plating rusted away, which is not the end of the world on the bigger boats (over 40 feet) that had 5/16" plating, but was certainly not acceptable on the smaller boats with 10 and 11 gauge (roughly 1/8") plating where the strength of the panel was pretty much shot.
At least one of the boats that I saw had the interior removed and had been partially replated, sandblasted and recoated. Two years later the rust was back and growing. The broker told me that the seller of that boat had spent more than his asking price to remove the interior, do the plate repairs, sandblast the interior of the steel and recoat it and was now frustrated that the rust was back so quickly. Even discounting for broker hype, the cost of replating a 20-30 year old boat is a very significant number.
My problem is not with the small amounts of routine maintenance that has been described above. That kind of work is no different than say maintaining teak oil on the deck trim. You get a routine, and simply do it and its done. My problem is with this insideous rusting of key structural components and connections, and the long term impact on the boat's strength.