To me they are very important. A salt-only or salt-and-pepper rub (or a salt, sugar, lots-of-paprika -based rub) epitomizes bland and after a bite or two I'm uninterested and moving on. Hundreds of subtle flavors are created during cooking/smoking which can be (should be, to me) highlighted, complemented and contrasted. I make rubs mostly specific to the meat I'm smoking and to the sauce(s) I expect to make for them. It is rare that I use a rub I've made for beef on, say, chicken.
As much I love sauces I like the meat to stand alone, without need for sauce. To me, non-existent or virtually non-existent (or heavily sweetened) rubs don't make this possible. Rubs do not need to be heavily applied, necessarily; their ingredients should be well-chosen and proportioned well and should be of quality. The recipe needn't be two-dozen ingredients long--a well-made rub can be fairly simple (though there are many rubs with many ingredients that I am quite fond of; by no means a requirement though).
But that's my palate and preference. Others may--and certainly do--feel otherwise. The world of food--the world--would be pretty boring if we all thought the same way.
(For me the least important ingredient is sugar. I don't use it in many rubs; when I do its proportionally small. I know several people who lean towards John's view of salt.
I make most rubs without any salt, preferring to salt the meat first then applying the rub over the salt. This allows control of the salt level irrespective of how much or how little rub is applied.)
My $.02.