While this is true and all, this really does make the case against the artists doing poor work on the weapons & armor in the D&D books worse. I mean seriously, the artist doesn't have to make the items, (s)he just has to draw them.
I know whenever I go into an illustration, more than 3/4 of the time I put into it is in research. If I'm drawing armor, I spend horrendous amounts of time pouring through various real armors of various types trying to find something that comes close enough to what I'm trying to do that I can use it as a strong, functional base for what I'm going to do. The same with weapons. Fantasy, modern, it's all the same. I don't go trotting willy-nilly into drawing a sword any more than I do drawing and M-16A2 with M203 attachment. You have to give it some thought.
But for me, it's a matter of pride. That's my work, and every piece I put out is a representative of my skill, how good of an artist I am. Granted they don't all say what I want them to say, but I like to think that they all say that I put in the work and time to have them not look like I made it up as I went along with nothing to base it on.