I've only seen it 3 times. First was when I installed the beta in the first place, second was after the last beta update with the test reset and the last time was when the game was released. I haven't seen any bug reports of players going through the tutorial more than once for unexplained reasons so you should be fine from now on.
The point of the tutorial is to get you acquainted with the mechanics and objectives for the game which apply to all the classes. The Assault was probably picked as the tutorial class because his gamestyle is similar to most other shooters. He shoots a gun, has a grenade launcher and throws out bombs.
I can't think of an FPS that did have a detailed tutorial that showed off each and every aspect of all the classes. TF2's only lets you play as the Soldier and briefly touches on the mechanics of the Engineer by showing his constructions pre-deployed in the gameworld. In my experience this is typical.
I don't see how this is irony.
Remember on Kotaku and some forum boards, where they said MNC was a total ripoff from TF2? Well, now we have a fusion of the 2 games.