The pitting of the camshaft lobes is attributed, at least in some cases, to corrosion. (I'm drawing that conclusion based on the fact that such damage is said to be seen in engines which don't get a lot of use).
Sticking valves are often the cause of cam lobe damage, the article says. I would think than in most automotive engines you would be able to notice a sticking valve, and you'd be having the valve train checked out asap... so you wouldn't need to rely on a UOA to show you cam lobe wear if a sticking valve was the cause...
Then another major cause is not enough ZDDP in engines with high lift cams, which need the barrier additive to protect after the hydrodynamic film of oil is breached. Here is where the cam lobes would not come apart necessarily in chunks, but they would score (like in the 4 ball wear test) and put iron particles into the oil which would not be caught by the filter, and presumably show up in a UOA. The pitting lobes (indicated in the article linked) give off chunks of metal that the oil filter traps, but seemingly not a lot of tiny particulate matter that you'd find in the oil itself.
Since we can't feasibly tear down an engine to assess how the lobes look, the UOA is better than nothing. We can at least see if indeed the lobe wear is in an advanced stage... or if the bearings are coming apart... if the UOA shows you that a cam lobe has reached the point that it is disintegrating, you can at least take the engine down then, rather than having something more catastrophic happen.
Dan
