I was re-reading The Elements of Style when I noticed a remark I had previously overlooked in the third rule of usage (Enclose parenthetic expressions between commas.).
The abbreviations etc., i.e., and e.g., the abbreviations for academic degrees, and titles that follow a name are parenthetic and should be punctuated accordingly.
Eg: Letters, packages, etc., should go here.
I understand why the other abbreviations are parenthetical, but am not convinced whether etc. should be parenthetic.
Etc. is parenthetical by definition: it provides incidental or supplementary information, not definitional. It means and other things, and if you wrote it out, you'd enclose the phrase in commas, wouldn't you? Whether it should be enclosed by commas in running text is a matter of style, and you should be guided by your manual of style, either the one you've chosen or one thrust upon you. You've apparently chosen Strunk and White. If it helps, the Chicago Manual of Style agrees with S&W, although it says the usage should be discouraged.
