I'm not sure of your issue here. Some classic utility breeds have a nice wide gene pool so we don't have to wish we could import new strains to improve it. Perceived lack of utility traits in those breeds can be managed through selective breeding practices to improve the desired utility, if people are that way inclined. For eg, Australorp, Langshan, Sussex, etc - all the classic dual-purpose breeds. It's not difficult to improve egglaying qualities of a breed - just time-consuming. You want to wait until the end of the birds' first laying season, at least, to find the best layers and breed from them. And breeding for taste or carcase size is even harder, due to the fact these are things that are only determined once the bird is killed, which rather means you can't breed from them
You need to be _very_ observant and find the visible markers toward the desired traits, and breed in hope from the birds that display them. (Weight is, of course, an easy one; but sheer weight isn't the same as "good breast meat" or "dark leg meat" or whatever).
Other utility breeds, such as Marans, Welsummer, or Cream Legbar, have a specific and unique-to-the-breed (ie a particular egg colour) utility trait that is too easily lost if outcrossing occurs, and the gene pool in Australia for the breed is too small, or too unreliable, for the average dedicated backyard breeder to deal with. This is where importation would help, by bringing in purebreeds with prime examples of the unique trait, that also fully match the standard for that breed - a standard, it is hoped, that contains the other utility traits of the breed. (ie, Marans cockerels should actually be quite heavy and have quite a dark meat, for their eating purposes).
Like all things in life, it's not an all-or-nothing proposition. Some people want to import to improve the physical looks of some birds - improved lacing, better feathering on legs, sheer physical size, introduction of a feather colour or pattern that doesn't exist here - and that's a perfectly good aim in its own right. Others want to import to find out what a previously-unknown breed tastes like. (I want Bresse for that purpose).