USING ALL UPPER CASE MAKES YOUR TEXT HARD TO READ. VISUALLY IT LOOKS AS THOUGH YOU ARE YELLING AT ME, AND IT’S MUCH HARDER TO READ.

Competent readers read more by identify the shapes of words rather than processing all the individual letters in the word. The ups and downs of lower case text form familiar shapes that we readily identify as words. 

Text in all upper case makes every word look like a similar block, and so readers are forced to slow down to decode the individual letters. Sentence case (upper and lower case) is much easier to read.

Novice writers sometimes write important messages in upper case, thinking they will be noticed. Setting out important messages in a way that makes them difficult to read makes no sense, and is perhaps irresponsible.