...cuz once a melody/rhythm has been identified as a copyrighted work, then it's protected and any violation is enforceable by the courts.

but what u're saying is akin to something i recall from Showbiz who said that when he samples a certain artist he's envisioning himself playing with those artist. it's how he shopped for records. so that if "so and so" was on drums, bass or keys or whatever...he'd look for those players as a way to put together his own band (i'm paraphrasing).

but again, it's difficult to single out those performances/parts from the original work. cuz it's the original work that's protected by copyright law.