Interesting. I went to the most recent Prerelease event for Magic: the Gathering and my first opponent told me to not use the D20 that came with the kit and to use his individual D6s. He said they had known biases. Maybe he was on to something.
Because the dice aren't evenly weighted, they have a strong bias. IIRC, Spindown D20s crit somewhere around 30% of the time.
He was probably on to something.
At least way better than spindown dice, which are known to be weighted.
Now, whether it matters if you use a weighted die to determine who goes first is a different story. If you're both using the same die, who cares?
I guess some cards use a dice roll, but that's pretty rare.
But the average MTG player at a con did not take stats in college, and not worth a fight. So ya just pull out another dice and smile and roll it.
The second more straightforward one is that the spindown dice vary from set to set and even within the set sometimes. As such the weighting is likely going to vary a lot.