OK Brian, here is my take on doing or recommending this type of repair. If your working on a car that has no value today or in the future and your only concerned about it lasting for a few months...go ahead, knock yourself out, plug the rust with fiberglass. If your working on a classic, say for example a 69 Mustang and the roof is rotted out...would you recommend filling the rust holes or repairing the rusted areas properly by replacing the rust with metal or maybe, depending on the amount of rust, the whole roof. In my opinion it's not a matter of "next bestest", it's a matter of bringing a classic back to it's original condition. If the owner doesn't have the knowledge or the equipment then I would suggest one of 2 options, either get the knowledge and the equipment or hire a person that can do the job properly. Too many a good, restorable vehicle, and we've all seen them, have been butchered to the point where a proper restoration isn't feasible anymore.
In my youth I made too many stupid mistakes with cars that today would be valuable, be damned if I'm going to pass on information to people remotely suggesting that fiberglass out of a can is an acceptable and respectable way to repair rust or restore a classic.
Just my thoughts.