Hmmm....
- With more usage it becomes increasingly irrelevant (small possible number of mixins vs large number of participants)
- If you are worried about someone correlating your "odd" mixin amount, simply do not use an odd amount. I think "noobs" are more likely to use the default than to choose some random number without understanding.
- Low, Medium, High examples are too high (probably subjective). Transactions start getting large pretty quickly (mixin * inputs).
With that said, I'm not necessarily against this proposal with different numbers (say 3-4, 5-7, 8-10 or something). It's just a wallet configuration after all. :) Maybe the "low" one shouldn't be variable, so you don't randomly pay a bit more fee than you "should" for routine transactions.