Thomas Shey
Legend
The 3d6 does certainly changes things a bit near the edges; you get chances of about 4.6%, 1.9%, and 0.5% for the next three spots where the d20 would have a 0% in the hard difficulties, and then some chances to miss on the other side for the easy difficulties where the d20 would 100% hit. (See the tables in the second spoiler below).
That was my point; it can actually handle low-incidence (but not trivial over large numbers of rolls) events. A D20 can't.
I mean, yes, you can weigh the bonuses to make probability more consistent--but it also requires you to limit the number of modifiers you can do because the rack up too fast. If you're starting at a 10, you can't really do more than 5 steps. There's still an argument for doing it (because it makes mods more weighty) but its not quite the same thing.