I would think it rather rare that a tappett would fail alone without a similar failure on the cam lobe...
Having said that, I do have a set of Cosworth L-1 cams from the mid 60's that I recently installed in a Lotus Twincam engine for a vintage racecar. Cosworth L-1's are rare but not all that uncommon. This particular pair are, however, unusual in that I have used them in several different engines over the years...each time with NEW tappetts!
All I can say is, contrary to "common knowledge", the camshaft lobes are still "as new"(profiled for me by Isky Cams).
It is always advisable to use new lifters on any new camshaft as, in my experience, it's the lifter that wears in to the cam lobe and not the other way round. I don't have all that much experience with pushrod engines, but intuitively, I don't see a big problem. If there is, I am sure someone will point it out!
Rod