Callouses are not a desired product of striking the makiwara-they serve no purpose other than making your hands ugly. Rope was originally used because it was cheap and available.
I remember my sensei looking at a students calloused hands and saying," Too much makiwara" I used to hit the makiwara a lot more than I do now and started to get the callouses so I backed off. Callouses can also be caused by poor striking technique.
My current sensei says you should use a round wooden pole and work all the strikes, single knuckle, spearhand, toe kicks, , knees, elbows,etc. Striking softly and gradually increasing the power. He doesn't have calloused hands. Now some, Higoanna as an example have really disfigured hands.
I remember reading the book,"Karate-Moving Zen" talking about students trying to get big ugly knuckles some by striking a concrete wall.
The important thing to remember is to train smart, start slow and build as experience and conditioning grow.

Duane