It is common, just not formal. Consider these: "I own no horse"/"I have no horse"/"I haven't got a horse"/"I got no horse"/"Ain't got no horse". The first is strikingly unnatural and formal, the last is way too informal for most uses.
"Чтоб" is slightly stylistically colored as an informal/old variant, and, of course it is freely used in poetry to keep meter right. It does not mostly sound rude in forums, it sound OK in literature (especially when writing "informal" dialogues). But it is a bad choice if you are writing an essay or some other piece of formal writing (a scientific paper, a legal document etc.)
Remember also that "Чтоб ты сдох/лопнул" is always like that. These type of wishes are not normally expressed "formally" with "чтобы"
Otherwise... For instance, I feel that I seldom ever use "чтоб". There is just no need to (except for "чтоб ты сдох"), and besides, sound much more fluid in speech.