The keyfile is mostly a file to help make your password more secure. A lot of people use crappy passwords. If they used the keyfile it would add lot's of random info to the password so if just the wallet is stolen they won't be able to brute force the password unless they also know the keyfile and have a copy. It's what truecrypt can use.
Strengthening probably addresses this better by making the bruteforce too slow to be effective against all but the dumbest passwords.
Its important to keep in mind that, recent hysteria notwithstanding, the greater risk to most bitcoin users is coin _loss_ not coin theft. Security measures are important, but if they make you more likely to lose your coins or suffer data corruption then they are probably a net harm to the users overall.
Basic wallet encryption is probably a net gain— widely used it should immunize the whole community against the creation of collection worms somewhat though it will cause some people to lose coins that wouldn't otherwise be lost. I doubt this is true for keyfile boosted encryption. Moreover, if you want that you can have it externally to bitcoin.
