I'm a former property insurance adjuster and I am still employed in the insurance business. My personal opinion, for what it's worth, is that all-risk is almost always worth the premium over named perils policies. I had to deny a few claims on named perils policies that would have been easily covered on all-risk.

Of course, as with anything, exceptions do exist; there are some commercial (and perhaps unique residential) risks that either simply do not qualify for an all-risk policy or are just very expensive that way. In that case, it seems best to go with the most comprehensive named perils form ("Special causes of loss," it's usually called) and buy any endorsements that you think are necessary for your situation. When it comes to commercial insurance, it's actually possible to buy named perils policy with special causes of loss form, coupled with a few endorsements and you end up with coverage that's actually more inclusive than the so called "all-risk" form.

In the industry we really shy away from using the "all risk" terminology because it doesn't cover all risk - it's usually called "Open Perils" or "Open Perils less exclusions" now.