It seems like you might be able to use them for two things, but both have some problems.
Concentration of Effect
If you have a chart of bad magical effects, say, and you order it in degree of badness. By using different dice types you can adjust the badness of your result. So, let's say your chart of bad effects looks like this:
- shrink 6 inches
- body odor
- hair loss
If you went from d4 to d30 it would go from 25% to 3.3%. If you were ambitious enough to have a 100 entry bad effects chart then stepping from d4 to d100 would change the chance of death from 25% to 1%, a pretty significant change.
I don't know how useful this is, because it's random, and just because you roll percentile, it doesn't mean your poor players won't roll a one. The concentrated effects are always possible, just less likely.
The other possibility I can think of for using Stepped Charts does deal with a kind of certainty:
Exclusion of Effects
If you roll a d4 on the badness chart you can never roll a 5. So, if we look at the chart from the other perspective we might segregate powerful/desired effects at the high end of the range.
Maybe your evil high priest has a Touch of Evil that he rolls on a table for. If you put the most devastating effects higher, you can limit the priest to smaller die types as he climbs in power. If death is entry 30, he can't even possibly kill someone with his touch until he works his way from d4->d6->d8->d10->d12->d20->d30.
The problem here is that uncertainty is still there; the EHP might have the chance of Death by Touch, but if he has to roll on a chart with 30 entries, it's only a 3.3% chance.
Maybe this would still be useful for mercurial domains like divination, or psionics, or wild magic. A psionicist growing in power would have new doors open up to her, new possibilities, but the powers of the mind would always be unpredictable.
Kind of a bummer it doesn't seem more clearly useful, but I thought of a different way to use charts that might be much more helpful to a DM in designing and playing on the fly. That next post . . .