That's what I took it to mean. As for probability, all I would consider is average results.
d4 = 2.5
d6 = 3.5
d8 = 4.5
Ok, that makes sense. That's an easy system to use.
I have a friend that would calculate the chance of rolling 16+ on a d8+d6+d4, but not me. If you need to defeat something's defense, bring all you got! If you do damage to yourself with a spell in a critical situation that's heroic and dramatic! Those are the best moments in the game.
But for spells where you are not trying to penetrate a defense, but are just trying to beat the difficulty of the spell, then you want to roll the minimum dice, right? For example, Spider Climb. If you wanted to cast this on yourself, then the difficulty is 1D4. So you would want to roll just enough to beat the D4 roll, to minimize the spell point cost. So in a non-combat situation, you would just roll 1D4, and if you fail, then keep trying until you beat it. But in a combat situation (where time was important), you might want to roll 1D6 or 2D4 to incerase your odds of beating the difficutly roll. Is this the way it works?