Dirt is right, not everything is a dominant. What defines chords like 9th, 11th, and 13th is there added harmony to the 7th chord (6/9 being the exception since it's a harmony on a 6th). Each actually stacks a new level of tension. 9th's are sort of airy, some tension but not as much as the 11th chord. 13th chord are really tense since the important intervals 6th, 7th, and root all collide. It's this color or quality of sound that makes us think of such chords. We know a 9th chord because of the way it sounds. That's what important about these chords is there sound, at least I think it should be. Even though a dominant chord (or other chord form) may exist within certain chords, it's important to draw some distinction between them rather than pushing them together by excluded or adding notes to make a new chord work. Myself, a composer would write a mb13 chord because I want the tension of the 5th, b6th, 7th, and root in one chord or broken. There are easier ways of writting that chord but it's that tension that makes me think of a 13 chord. Altered chords are nothing more than sub'in a chord with a chord outside the key. A perfect and common example is secondary dominants.