Things that are not a good idea:
(1) Using the same password for everything
(2) Playing golf in a thunderstorm
(3) Getting involved in a land war in Asia
(4) TYPING EMAILS IN ALL CAPS
But why? There are all kinds of reasons out there, things such as, it is not good 'netiquette' or it comes across as aggressive and is considered SHOUTING! Although some may consider one way 'right' and another way 'wrong,' I am not willing to take this route. As a little history lesson, it is interesting to note that:
Manual typesetters kept them [capital letters] in the upper drawers of a desk or in the upper type case, while keeping the more frequently used minuscule letters in the lower type case. This practice might date back to Johannes Gutenberg.
"Capital and small letters are differentiated in the Roman, Greek, Glagolitic, Cyrillic and Armenian alphabets. Most writing systems (such as those used in Georgian, Arabic, Hebrew, and Devanagari) make no distinction between capital and lowercase letters (and, of course, logographic writing systems such as Chinese have no "letters" at all). Indeed, even European languages did not make this distinction before about 1300; both majuscule and minuscule letters existed, but a given text would use either one or the other."
So, as you can see the caps issue was a neutral one. I think the best reason that typing in ALL CAPS is a bad idea is simply an issue of effective communication. Peter Bilak says it best with this quote:
"Right and wrong do not exist in graphic design. There is only effective and non-effective communication."
So while folks have…
(1) Used the same password for everything
(2) Played golf in a thunderstorm
(3) Gotten themselves involved in a land war in Asia
(4) TYPED IN ALL CAPS
…they may not be the best ideas and should be avoided.