I agree with your general point hdiddy, but I was referring to a particular type of situation, which is probably most common: You get called for a gig a at restaurant to play some standards with a trio you've never played with. You go to the gig and the leader calls "I'm Getting Sentimental Over You." You can't dig into your bass cover for a written out line you made for that tune and play it that way. You'll never get called back, and you don't have the time between tunes usually to be looking for music. Even if you bring lead sheets or a real book, people will most likely look down on you (whether or not it's right).
Also, I don't think any standards are played out. In fact, I saw Steve Kuhn with Ron Carter and Victor Lewis (if I remember correctly) a few months ago play "Blue Bossa" at Birdland and it was cooking! My point is that even a played out standard like that can be done in a fresh and interesting way.
Now the whole what is or isn't jazz thing is a separate issue. If we get into that we'll be here all day!

But I can say that in my playing experience it's very rare to be given a completely orchestrated part for a jazz band, although it can happen. Even for most big band charts you will find written sections and chord changes.
I don't think you can separate jazz from improvisation. If you want to be a jazz musician, you must know how to improvise. There's no question about it.