If we are talking about reality, and I am using learning software development as a benchmark here.
I think we can safely break skill levels into novice, journeyman, accomplished, expert, master and grand-master. Now, the gap between the first three can be staggering. However, after that , people specialize. It's impossible to be a master or grandmaster at everything (save for a few exceptional ones). From what I observe, it is usually a bell curve. A novice programmer takes a week to do what a journeyman can do in a few days, an accomplished developer takes one day, while an expert takes a few hours.
I don't know if this applies to other fields, though.
Now, a linear dice mechanic doesn't mean that that a journeyman is linearly more accomplished than the novice. If I assign the rung as such -- novice gives a +1 to the dice roll, journeyman +3, accomplished +6 and expert +8, there is sort of a curve there. A linear dice mechanic just means that a +1 increases the chance of success by 5% more increment (but as others may point out, the improvement is more than 5%. If you have a roll a d20 and need to roll a 1 to success, having a +1 bonus means your chance of success has doubled).
A linear curve is easier to figure out the probability of success. If I need to roll below 13 on a d20, I figure out that my chance of success is 60%. If I have to roll below 2d10, then it's harder for me to calculate the probability.