The third way: apostophes as visual separators

Today's entry for the Common Errors in English Usage E-calendar was about plural forms of abbreviations:

RBI/RBIs

Some people reason that since “RBI” stands for “runs batted in,” there is no need for an additional S to indicate a plural, and speak of “120 RBI.” However, though somewhat illogical, it is standard to treat the initialism as a word and say “RBIs.” In writing, one can add an optional apostrophe: “RBI’s.” Definitely nonstandard is the logical but weird “RsBI.”

The same pattern applies to other such plural initialisms as “WMDs” (“weapons of mass destruction”), “POWs” (“prisoners of war”), and “MREs” (“meals ready to eat”); but “RPMs” (“revolutions per minute”) is less widely accepted.

This prompted a reader to pose this question on a different point:

I read: “In writing, one can add an optional apostrophe: “RBI’s.” and wondered:

What does the apostrophe signify?  Is it a contraction?  A possessive?  Is there another use for an apostrophe?

I have seen this before but I don’t understand it.  Can you explain?

This brings up a very interesting point, since the apostrophe in this case isn't a contraction, nor is it an example of an abbreviation's possessive form. In this instance, it is merely a visual separator to avoid possible confusion. I myself would not use the apostrophe in this case, but there are many style guides that would back me up if I chose to do so.

"RBI's" vs. "RBIs" is not the best example of what I'm talking about, and neither is the tradition, in some style guides, of endorsing the use of an apostrophe for decades, as in "the 1980's." I don't care for that usage, either. I would go with "RBIs" and "1980s."

It's difficult to be entirely consistent, however. There is one case where I like the apostrophe used as a visual separator: when using the plural form of letters of the alphabet. Take this sentence as an extreme example:

"A's are harder to get than C's."

If I drop the senseless apostrophe (it's neither a contraction nor a possessive), I get this:

"As are harder to get than Cs."

Because "as" itself is a word, the apostrophe actually helps avoid any hiccup the reader may experience trying to decipher the opening of that sentence. Having committed to the apostrophe in the plural for "A," it's hard to drop it for the plural of "C" in that sentence. That would break my own style guide rule: Avoid things that look strange.