Not necessarily, for that very reason. In some cases, you're going to want to make the engravings permanent. If you happen to put it on a level where those guys appear, they'll end up walking over it and ruining the engraving if it's in the dust.
It's only permanent if you use a wand of fire or lightning. All other methods of engraving can erode 1 turn after you wrote it, if you're unlucky.
Erode happens via passage of time, and is accelerated by:
1. You standing still on it.
2. You shooting/casting/melee attacking on it.
It's permanent if and only if you levitate on it before it erodes.
Dusting is used when you can't defeat them. Write 10 in a row, drop something on it and run if you can. Otherwise, just keep adding one E-word every turn. Erode happens slowly, if you keep writing, you will have some of E-words ready.
If you dust enough you can even melee on it. Write 10 (always check if it's successful or not and how many are still intact), melee once/shoot twice, write another 10. The buffer for engraving on a single tile is huge.
The point is you can use dusting right now, and it will work immediately. Engraving takes time, and if it erodes before you're done, you have to start over. Engraving may work if you have a lot of time to prepare ahead, e.g. in front of Sokoban's zoo. Not in most places. Engraving more than one characters at a time, as I've stated, will disable any E-word already existing on that tile, and you will be stuck there for a few turns, even when monsters are hitting you. You can't cancel and the game won't interrupt you.
8. True, but the 'E' word is still useful for filtering out melee or cursing attacks against enemies who respect the 'E' word (liches come to mind).
Magic resistance is the only thing to save you from liches. E-word only prevents a melee attack, which a lich seldom uses. After a very short time they will no longer be scared and will cast spells/shoot as usual.