So far they haven't been that prevalent. I don't want to overoptimize this too much. It takes time to rewire one's muscle memory, and needing shift for some digits but not others smacks of over-cleverness. There isn't that much difference between the frequency of square brackets and digits. Parens on the other hand are easily an order of magnitude more common.
Yes, you're right. I had misread what you wrote earlier and thought that you simply overloaded the [] keys with (). That you actually swapped them is a fine solution indeed.
