I think that you are missing my point. I'm saying that not being arbitrary is too much to ask for from a practically useful system of measure, but what the metric system has going for it is that it is at least consistent with itself.
> but it's hard to escape the impression that metric is an arbitrary system trying to retrofit claims of non-arbitrariness.
I'd like to see any of these claims of non-arbitrariness. I have managed to escape them. The point of redefining the unit in terms of some universal constant is not to avoid arbitrariness but to avoid the inherent volatility of defining it in terms of something which is not constant and not easily reproducible outside earth. No one is arguing that 1/29979245 isn't a completely arbitrary number.
Granted, I don't really care much one way or the other -- I think metric is more useful for scientific/mathematical/engineering/etc. applications and some of the old imperial units are more useful for things like cooking, and tend to recommend people use whatever's most practical for their task -- but it's hard to escape the impression that metric is an arbitrary system trying to retrofit claims of non-arbitrariness.