S. McCloud, in Making Comics (p. 156), gives reasons to use or not use all caps in comic-book lettering.
Pro
- About 98% of all English-language comics in the last 100 years have used it, including nearly all of the comics now considered classics. If it ain’t broke, why fix it?
- Capital letters are easier to letter by hand.
- Caps fill the space more efficiently [that is, less efficiently – fewer characters use more space].
- Caps blend better with pictures.
- Caps look better with frequent bold/italic type.
Con
- There are a lot of things comics have rarely done in the last 100 years, including nature themes, subtle characterization, and sophisticated artwork; that’s no reason not to try them.
- One of the most popular comics in history, Tintin, uses upper- and lower-case lettering, as do other European comics, and it looks great.
- Easier doesn’t equal better.
- A little whitespace never hurt anyone.
- If upper- and lower-case letters don’t blend with pictures, how do we explain five centuries of illustrated books?
- Bold type is overused and melodramatic [not that we were talking about that].
Missing from this list: Everything else you read all day is in mixed case, including every other book you have ever read.