The guys were able to pull of this feat, not because they had never been constrained by the rules of the music they played, but because they knew them so well, were so intimately in tune with those rules, that they could transcend them without the whole thing descending into the mess that would transpire, had they been lesser 'masters' of the art they conveyed.
Similarly in the world of painting, abandoning the forms of traditional figurative work and moving into the abstact is by far most sucessfully accomplished by those who have mastered the tecniques of drawing, painting and composition demanted by represantational work and taught within the framework of a formal art education. Rarely does a genius emerge who has not cut his teeth with a traditional training to support his venture into the more experimental side of the work.
In my oppinion, no less is this is true when one comes to consider poetry. Contrary to what many people believe, there is a structure of 'rules' [bad word - tecniques would be better] that is employed in order to give poetic works 'euphony' [a good sound], tecniques which have been developed over centuries and which I think should be thoroughly studied before one even begins to think about creating works of ones own. The tecniques of 'scanning' and 'rhyming schemes' can be utilised to analyse how given poems work their magic, and it is by absolute familiarity with these tecniques that the skill can be built to begin works of ones own. Failure to learn these 'rules' will like as not result in the production of 'doggerel' rather than peotry [though the line between the two can be thin and a matter of personal taste], and although one will not see it as such at the time of writting, a chance encounter with the work a few years later will soon expose it in all it's gory detail [Knowledge won by hard experience I hasten to add

I believe it is only when one is absolutely familiar with the 'rules', to the point where one could use them almost as second nature rather than with conciouss thought, that one is ready to embark on the very perilous venture known as 'free-verse'. Free-verse is poetry that does not follow the rules of poetry; it is the 'free-jazz' of poetry, poetry that marches to it's own drum - or indeed no drum at all. But never the less, as with the examples I gave above, free-verse only works when it rises above the constraints of the 'rules' upon which poetry is based and not when it is written without knowledge of them at all.