I am a huge supporter of Krown. However, it really comes down to what you want out of the car....
If it's a new(ish) car that you're going to run for a few years and then dump or trade in, then yeah, forget about it because even without any rustproofing rust won't be visible anywhere when you get rid of the car anyway.
If it's a decent condition older car that you want to drive for a few years and dump, then I'd give it some consideration. The car is likely getting to the age where rust could become an issue, so for $100, why not get it sprayed....Peace of mind knowing that components like the brake/fuel lines and such are protected. Plus, when you go to resell a non-rusty car will return much better resale.
If your someone like me who's relationship with some cars spans decades, not years, then it's a no-brainer. Once a car has started to rust, it'll never be the same, so I do everything I can to keep it at bay....When I dailied my Sonoma I used to get 2 sprays annually (in the fall, and in the spring), plus I'd by the cans at CT and do touch-ups to the spray every couple weeks....Obsessive yes! Now I still get my dailies sprayed annually, and do monthly touch-ups with the cans....Currently daily is the 96 Firebird, it's seen a LOT of salt and I'd challenge anyone to try to find a spot of rust anywhere on it...Except for some surface rust on the two rear springs; I had installed them before last winter, after I had it Krown sprayed, so they went unprotected for a full year.