This particular "Tetris Don't" is something of a variation on The Left Wing or the Right? - except here the player chooses both wings. It actually tends to occur with greedy novices starving for Tetrises. Sometimes, in their foolish youth, they accidentally leave a 3-block gap open on both sides - somewhat better explained in the Flash example (left). Sure, a straight piece will fill up one 'alley', but before they know it, there are no straight pieces to be found, and it becomes even tougher to manage. Obviously, the straight tetrad doesn't need the extra stress in accomplishing an additional tetrad-specific responsibility.
To prevent this wholly unnecessary situation, start off by placing the first block in play flush against a wall. Don't start in the middle. Also, see Staircases for further tips.