Did Guinan EVER did or said something useful besides running the bar??
She could form a sentence.
Yes. She does do a lot of useful things in the series. When she sensed that the time line had changed in "Yesterdays Enterprise", she immediately brought it to Picard's attention in the best she could. She didn't play the mystery role by telling Picard "Hey, things are different. You should figure it out." And even though she didn't bring up the Borg in "Q Who" until the Enterprise encountered them, she did warn Picard to leave before that would happen. Would telling Picard about the Borg make any difference since Picard's mindset to any creature in the galaxy was "We are ready to encounter it!"?
More importantly, she says things that most heroes (and even writers) never get to hear. In "The Best of Both Worlds Par 1", Picard actually contemplates on the thought of the Federation losing this battle and it resulting in the end of humanity.
GUINAN: This isn't the end.
PICARD: You say that with remarkable assuredness.
GUINAN: With experience. When the Borg destroyed my world, my people scattered throughout the universe. We survived. As will humanity survive. As long as there's a handful of you to keep the spirit alive, you will prevail. Even if it takes a millennium.
It's a great line that never seems to hold any kind of relevance to stories like this one. It's all about doing what it takes to prevent something that humanity occupies as if that's the "be all end all" thing of our existence. Even if the Borg did destroy Earth, humanity still has other worlds, other allies and the potential to be great again.
I also don't think her role as bartender demeans her character in the slightest. It just happens to be a job that she enjoys doing, and what better job is there for a species of "listeners" to have?