One problem is totally specific to baseball. After decades of saying "51 RBI's," some gurus thought they would change it to "51 RBI." Their reasoning seems to be that RBI can mean runs batted in; so it doesn't need a plural indication. Now announcers waffle back and forth and even pause and slide as they worry about this issue. One was so confused the other day he uttered something like, "He has 6 RBI and 2 home run in this series." I wish they'd worry about other difficulties more than they do. Here's the skinny: If RBI is used, it is an abbreviation like K for strikeout(s). You wouldn't say, "He has 10 K"; it's, "He has 10 K's." (10 K is a footrace, isn't it?) Thus, when the abbreviation for RBI, which means either run batted in or runs batted in, is used, the plural is RBI's. Oh, if you have a problem with the apostrophe, it is appropriate for the plural of numbers, letters, and symbols. If the abbreviation is written with capitals, I accept such as RBIs. Clarity is of utmost importance. Thus the plural of the letter "a" needs to be "a's," not "as." How about, "I made two As, three Bs, one C, and no Ds or Fs"? It appears quite clear to me, although I tend to prefer the use of apostrophes here.
As a footnote to the RBI discussion, our newspaper uses singular-looking abbreviations for various baseball terms: AB (at bats), R (runs), H (hits), BI (runs batted in), BB (bases on balls [walks]), SO (strike outs), 2B (doubles), etc. Exception: for a later listing of runs batted in, the boxscore prints RBIs. Why the inconsistency? Your guess is as good as mine. These abbreviations are to help us find the facts as quickly and in as little space as possible. We know the number of, say, homers may be one or more. We need not write more than HR--Smith 2 (10), meaning Smith hit two home runs and now has ten for the season. Likewise, RBI--Smith 4 (41) expresses that Smith had four runs batted in during the game and now has forty-one this season. RBIs--Smith 4 (41) costs the newspaper one unnecessary letter. In summary, choose
"runs batted in" and "RBI's" (or "RBIs") for speaking and writing, but "RBI" (or "BI") for cryptic headings or notations as in boxscores.