You make a good point of course. I'll set a point off the mark in some cases.
This particular marker is not a channel marker. I guess I simplified things too much. It is actually in front of an island that is in front of the channel.
I know where the island, channel, marks etc are since we sail there several times a week.
I just thought that setting a way point and missing it on purpose was a creative use for a GPS.
Not taking about normal navigation.
As to the idea that setting a point off the mark is safer than setting a point on the mark, I'll defer to folks that know better but:
1. The marks can be off quite a bit from the chart
2. The GPS can be off quite a bit
3. The channel can be moved a bit as dredging is done.
4. The channel can be as narrow as the margin of error for 1 though 3.
5. You can come in at an angle so your course is through the mark to your offset position.
Since 1-5 all are true whatever number you put in the GPS for a mark is likely to cause you hit the mark if you don't have a proper lookout. It is just a matter of time.
So the question is? Is it worth while to put in an off-set mark? I suspect like most things marine, sometimes yes sometimes no.
In either event I suspect a lookout is the best precaution.