Yeah, IMO most modern cars are well enough protected from the factory that you don't need aftermarket undercoating. And even with older cars (lots of bare metal and other rust-prone areas) you always had to watch that the undercoating was done right or else it could *cause* corrosion problems. I've had plenty of rust repairs done on "rust-proofed" cars over the years.
I do, however, still get certain older cars oil-sprayed in vulnerable areas. Sorta messy, but it works well. Inside the doors, etc. can be a good idea as long as it doesn't plug the drain holes or get on things it shouldn't.
But in any event, you gotta clean 'em. Just look at how many cars are still rusted out around the wheelwell lips, for instance.