The fine lines in the roller ball pens are part of the attraction.
The multiplicity of colors is great, too.
I use both the highlighter types and the pens. I tested all the colors by scribbling heavily on white fabric, then ironing. There was a faint whitish remnant of the marks left, and when I froze the fabric, some of the color came back.
I hand-scrubbed the fabric briefly with a little detergent and pressed it dry and all of it was gone without a trace, even after re-freezing it.