It comes down to clarity in the end, really.
You want the posts you read to be clear. Otherwise, the way the poster says things gets in the way of what they're actually saying.
It's pretty context dependent, eg if you write in Icelandic on an English webboard, you're not being clear, even if your Icelandic writing style would bring tears of gratitude to the eyes of a native speaker.
It seems to me that the vast majority of mumsnetters are most comfortable reading standard English prose.
Sooooo, if that's the style that's most comfortable for most of us:
posting all in capitals = less clear.
300 word post with no full stops = less clear.
posting in textspeak = less clear.
unorthodox spelling = less clear.
It's not a snob thing, let alone intelligence: it's just about communicating as clearly as you can.
[welliemum climbs down off soapbox]
You may talk amongst yourselves now......