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Ian,
People have used them without incident...and in my opinionthey are okay provided that the "tip" end (part that goes on the optics) is clean and grit free.
I don't use one myself preferring to use Q-tips/ cleaning fluid for big cleans and/or the occassional optics wipe in the field but that aside, a lenspen in good condition just make sure to "blow" off the dust from the optic surface (and also the pen) and don't press too hard.
Cheers,
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