IIRC, that pack of Milwaukee markers had 4 in it, for about $6? Though I do like the idea of retractable.
Adding the initials goes back to the days when I first started in the assembly area. It was more about which machine it was made on, rather than "who" did it. That way, if we ever had a squareness issue, we would know where it came from, since there were 2 large presses. Even a tiny amount can be exaggerated over long runs of upper cabinets, especially with frameless.
I just got into the habit of putting the date with it too.
It's kind of cool to find that on the back or inside of things when they get removed due to damage repair or for an up-date/remodel. In the commercial arena, this happens more frequently than you would think. Wings get remodeled or added to and once in a while there is water damage to a sinkbase and they get replaced. I have seen things taken out in as little as a year, and others well over 10 years, with my scrawl on the back.