Here is why I think musicians who make smooth jazz are legitimate.
Lets compare various aspects of smooth jazz to a genre of jazz that no one would argue is legitimate jazz - Bebop.

Bebop was based on popular music of the day. Sorry revisionists! Beboppers were only one or two steps removed from the popular music textures of the day. All of the instruments in bebop were instruments that the average shmoe were familiar and accustomed to, acoustic bass, piano, horns et

c etc. The forms were borrowed from pop tunes of the day, Honeysuckle Rose, Back Home In Indiana, Ive Got Rythm, blues forms etc etc. Except for an athletic "attitude" and inherent hippness applied to it, bebop was something that was hip because a fresh spirit was brought to the music that was, albeit "quirky" to many at the time, relevent to many in the culture. When a person of the day heard bebop, he was probably amused at the sensabilities exhibited, and maybe even questioned the players intentions, but certainly did not hear jazz as some foreign abstract, esoteric mish mash that he did not understand. He may or may not have dug it, but he respected and understood it.
Mainstream jazz today, reflects sensabilities of the musicians in the circle, but for most, totally irellevent to the culture of today. The typical sounds of mainstream jazz do not reflect the instruments, the grooves, and the forms that are prevelent in the greater culture (non-musicians), as bebop did back in the day.
Smooth jazzers today, it could be argued, at least are reflecting the musical sensabilities that are prevelent in the greater culture- funk and hip-hoppy grooves, electric instruments, a certain contemporary harmonic vernacular etc etc.

They do this all while improvising the snot out of it, exhibiting a high level of proficiency, musicality and experience at the same time.
It could be said that smooth jazz is carying that tradition of improvisational music on forms based, or relevent to, the pop culture, something that was left by the wayside by hard core jazzers many years ago. Not that hardcore jazz is not cool, and even necessary, it simply stopped being a vehicle that epresses contemporary sensabilities to the average non-musician on the street.
The label "jazz" is a two edged sword, it suggests a wonderful tradition and lineage on one hand, yet pigeon holes many minds as to what the significance of it is on the other. Perhaps the sad part about it is, that perhaps "jazz" as an art form that was both explorative, innovative, relevent, AND popular to the culture as a whole, was a special genre unique to its own time and space in history, that will doom many who continue use labels without a thorough analysis of its significance.

J