I've only pulled one 'sickie' in my entire working life (school was a very different matter and I won't even mention uni!) and that was more of a protest about crewing levels than anything else. I'd reached my limit and felt atrocious, all due to apalling rostering and not enough pilots. I could have just said that I was fatigued, but that would have opened a can of worms.
Usually I wouldn't contemplate it since it means someone else is getting inconvenienced and it can stuff the roster for a few days if the company is tight for crew. The only one's who suffer are the crews in that case. A bit selfish really.