I always rather assume that posts in lower case are written by coackroaches. As in "archie and mehitabel" by Dom Marquis, where archie, the reincarnated journalist, who came back as a cockroach (presumably because he had been a good journalist and deserved promotion) used to type by diving at the keys. But he couldn't manage the shift key, which was stiffer.

Mind, with computer keyboards, that wouldn't apply would it? The poet e.e.cummings did it too. I don't know if he was a cockroach as well.

Since it makes it harder to read, I'd say it counts as being discourteous, unless you have similar problems to archie, as some people do who aren't cockroaches. Running the words together is doubly discourteous, with the same proviso. Still, it's not important enough to make a fuss about, and to do so could even encourage some people to faff around like this.