I have been thinking about this when using different dice.
Part of the fun is that the tn is a number that is meaningful in superstitions and what not so what about the numbers 7 and 4?
If you use a d10, you can make the target be 7.
If you use a d6, then the target is 4. Think of those dice that have blue pips except for numbers 1 and 4. Another benefit of the d6 is using dice pools.
Anyways, what a smart little mechanic.