Your points are interesting: I think I will now get some more practice at using 7ths and 9ths, resolving them in different ways, not resolving them, and trying to become more familier with their sounds and effect. I always think that, when unresolved, 7ths (besides dominant and diminished 7ths) make things sound more relaxed, and add a 'peaceful' colour to the music. 9ths have a similar effect (though minor 9ths are horrible in my opinion
).
I'm don't think they strengthen the tonal system, as the tonal system is based on tension and realease, and 7ths create (though very very mild) tension, and not resolving them is robbing the listener of the 'release' that resolving them would give. Thus, I think they weaken the tonal system - and I'm all for it
As for counterpoint - I think counterpoint applies to all music, whatever harmonic rules apply. Counterpoint (to me) is more about the way the parts interact than what the notes are. For example - counterpoint is equally important in the works of Bach, and of Bartok - Bartok certainly doesn't follow the same rules when it comes to treatment of dissonance, and harmony in general -but counterpoint is there and is jsut as important as it is in the Bach. So, I think counterpoint is not affected by unresolved 7ths and 9ths. unless of course you were refering only to Counterpoint as it was used in the Baroque, in which cae all of what I've said is utter crap
