I'm pretty sure in the Tin Pan Alley were some songwriters who never played an instrument. Jim Morrison never played anything, and yet, what a brilliant songwriter he was. Absolutely, I'd add there are actually three disciplines: musicianship, songwriting, and improvisation.
You can be a brilliant classical musician, but never be able to improvise a single note. You can have rudimentary knowledge about playing but improvise like God. Or you can have no both skills and still compose brilliant melodies. Mark, however, excels in all three disciplines and hence the diamond of the industry...
Do you really think Mark is a great improviser? That's interesting, as much as I love him and his music I always felt his playing was somewhat scripted and extremely polished, more akin to your classical musician analogy.
I'm not saying mk is a poor improviser, I'm just not sure he excells in that department.
Really interesting opinion though....
Well, I know a thing or two about this, as I'm exactly the first type. I can't improvise at all, and throughout my life, I spent a lot of time and money trying to learn this craft and never succeeded. I gave up on learning improvising, and I've met quite a lot of people who may be playing not as advanced as me, but they are really good improvisers. I, however, seem like have a complete improvisation block.
Mark is a brilliant improviser. I studied a lot of his playing and surely he's as talented as a songwriter as an improviser. He's a brilliant guitar player overall. Maybe he's not technically mind-blowing, he's not Steve Vai or something, but overall he's amazing.
Even "scripted" improvisation is also not easy. In fact, all improvisation is scripted anyway... Like the way we speak, we speak with words and letters that are already known, it's just a new arrangement of those that we make up on the spot and it's called improvisation. I improvise this post right now.
Mark, in his own songs, taught himself a very unique style of improvisation, the way he uses intervals, certain traits of his sound, it's not all about fingers and "touch". That's just another little thing that tribute bands can't recreate. He's an excellent improviser for his songs, he's an excellent singer for his songs and the best rhythm guitar player for his songs and the best lead guitar player for his songs. That's why he's so unique and practically uncopyable.