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Should these be discarded in favor of a traditional stamped cancellation mark?
In my opinion, yes. Anyone can make a pen-cancel. On OLDER stamps, pen-cancels are indicative of revenue usage, and usually not postal usage.
Of course, there really are some people who collect pen/Sharpie cancels of modern stamps. Some serious, some out of humor, some simply because they are more easily found on mail than normal cancels.

Also, on a very few classic era stamps, certain pen-cancels actually command a premium. These pen-cancels were the equivalent of "fancy cancels" and were often used by specific postmasters. An example of this is some of the pen-cancels of the Finland serpentines.
Have fun with your Machins!

k