Mike, no need to be dubious of the efficacy of adding mycorrhizal fungi to soil. What you say is true, but I reccomend that you view a few more studies on the subject. There are many interesting peer reviewed papers on the effects of innoculation with vesicular arbuscular mycorrhizae, or ectomycorrhiza. An internet search through public databases and even Google will produce several, so there is no need to cite them here.
You can also note that in the experiments both an inoculated and sterile group are used, and the inoculated groups often show over a 50 percent gain in growth over the sterile group.
Mycelia are almost everywhere in the soil and, as you said, they will come. But many areas that are disturbed due to development or logging have had an upset to the natural balance of fungi that may result in a decrease to the natural rate of infection of mycorrhizae.
Without question, it is better to innoculate than to not. Whether one should use native or non-native species is a separate discussion. Isolation of species is not a huge problem either, as there are sources to obtain pure strains of mycelia--it's just a matter of knowing which ones can resist competition from the fungi that may be already established in the soil.
Lucas