As with topic #21471, why are we lumping fountain/nib pen and ballpoint pen drawings together?
First, having pens classified primarily by a couple subtypes is pretty dumb imo. If you look through these, many are quite difficult to distinguish visually. Its made harder by the fact that a good deal of these works have nothing to reference for size and are photographed/scanned with somewhere between mediocre and crappy quality. Sorting posts into anything narrower than pen requires either the artist to confirm or a tagger to confirm that the post was created a particular type of pen. Visual categorization relies on looking at the properties of the pen marks. While this works to some effect, it's still quite vague for a majority of posts. Combine that with some amount of laziness/inattentiveness results in tags that are quite messy.
As pens have advanced, there are also an increasing number of types of pens which are often difficult to distinguish from one another. New types of pens, such as gel pens, rollerball pens, felt tip pens, plastic tip pens, and brush pens (more marker), along with advances in the technology of each type of pen also make them increasingly difficult to distinguish from one another.
Depending on who you ask, the specific definitions of millipen and ballpoint pen are also different. For ballpoint pen, the definition can range from "a pen the uses oil based ink and requires the user to apply pressure in order to make marks on paper, which leaves an indent where the pen traveled and a slightly glossy finish" to "any pen that has a ball mechanism (as opposed to a brush, nib, or felt tip)". Same with millipen. The only (from what I can tell) real qualification to be a millipen is to make consistent thin and even marks. Many different pens are capable of making marks that fit the bill. Some can fit under both definitions of ballpoint pen, while others are plastic or felt-tipped but achieve an identical effect. "Thin" is also problematic since it can be difficult to gauge the scale of a work as mentioned above.
I think that the last 2 tags should be used sparingly when its fairly clear that a particular pen was used and I'm in favor of having pen (medium) as a large overarching tag which makes more sense than having some separate tags. Frankly I'm also in favor of nuking or aliasing the last 2 tags.