I don't get why people call post hardcore screamo though. That one escapes me.
Because they are idiots? I dunno, but there is little to no emo in the classic ATDI albums (mainly social/political commentary when you can pull apart the lyrics), Blood Brothers (mainly social critique as well as critique of certain types of individuals), or BearvsShark (critique of satisfaction with the suburban lifestyle, among other things). Trail of Dead, if they're post-hardcore, are sometimes a little emo-ey, but there's also a lot of social commentary in many of their songs as well. So yeah. Screamo is not equivalent to post-hardcore. People are stupid.
Harsh vocals are good, when used appropriately. A lot of people can't see the emotion in it though.
Harsh vocals can be VERY effective. And not just for conveying anger. Examples abound, but I find a lot of screaming often can convey a particular feeling of desperation that is difficult to convey otherwise (there is a difference between quiet, controlled desperation a la My Bloody Valentine and screaming, chaotic desperation a la Jawbreaker, for example).
A lot of people are just put off by discord in general, which is why harsh vocals (like harsh guitars, bass, drums, or whatever) are often shunned by your law-abiding, tax-paying, puppy-dog-petting Coldplay fans. But fuck that shit. Life isn't always melodic and symphonic, and discordant rock (be it hardcore, discordant post-punk, post-hardcore, various kinds of metal, screamcore, or whatever) reflects that both lyrically and musically. And isn't that the point? Music is supposed to convey a feeling, emotion, or idea, and the idea that all is not well in the world is just as valid (if not more valid) as the idea that we should send Chris Martin more money.