Those of us who dislike rap are not arguing that rapping takes skill, we are arguing that it takes immensely less skill than many other forms of music.
To be a great "rapper", what skills do you need?
You probably have to know a thing or two about rhythm, but I seriously doubt you need to know EVERYTHING about rhythm (simple & compound time, polyrhythms, complex cycles, syncopation, etc) and be able to play anything (switch meters with ease, etc).
You have to be able to write lyrics. So, yes, they are poets. I'll give them that. Do I think they are comparable to the best song writers (Great American Song Book, and the likes of Billy Joel, etc), No. Few have the knowledge (yes, I did just say that).
Melody. Here is where the skills cease to exist in rap. I HIGHLY doubt that there is a single rapper out there that has mastered melody alone. (can sing every mode (major modes, minor modes, melodic minor modes) along with all the other different scales (altered scales, chord scales, Indian and other 'regional' scales)).
Harmony. Here is something that is almost never found in rap music. Harmony is even MORE complex than melody. You have triads, tetrads, etc. You have counterpoint. You have enough theory and craft to cover to spend a lifetime mastering.
Improvisation. This is what REALLY sets apart Jazz from anything else. Great Jazz artists take ALL the skills I've listed bellow (with some others, too) and use it to create music on the fly. It takes many many years of practice, study, experience, skill, and talent to improvise well over any conceivable piece of music. Most of the greats spend most waking hours practicing their skill.
Composition. Again, Rap doesn't compare to any great Jazz or Classical music. Jazz and Classical compositions are far more complex than anything you will ever find in Rap and it takes far more knowledge and skill to be able to understand it and compose music in those genres.
On top of just the knowledge, most of the great Jazz and Classical artists are proficient in more than one instrument. How many rappers do you know that are proficient in many instruments (including experts in at least one)?
It simply doesn't take as much skill to be a great rapper as it does to be a great in many other genres. It's not even close.