Hey Bob,
The phrasing 'has no fatigue limit', while accurate, might be a bit confusing (I had to parse it 2 or 3 times to get your meaning).
How about, 'aluminum always has a limited fatigue life, regardless of stress, unlike steel, which has unlimited fatigue life if kept under its fatigue limit'?
Having said that, isn't it an argument that's made without enough conditions attached? For example, RV spars are not only aluminum, they are anodized (not alodined), which, if I've been correctly informed, halves their fatigue life. No one panics about that, because the number of cycles required to reach life limit exceeds the total likely to be achieved in a typical RV's lifespan. (That may change when hours on RV's start approaching those of some civil/military a/c that have had fatigue failures...)
From earlier posts: I understand the concern with galling with aluminum on aluminum, but isn't that more or less limited to pipe threads? Otherwise, we'd be having galling problems with all our aluminum flair fittings.
Charlie