Is that just a general feeling or have you witnessed or encountered this yourself?

My take on posting material online (which seems to be a thing you have to do these days if you're hoping to get yourself noticed) is that it's got a date and time stamp, so if plagiarism does happen, at least it's one more scrap added to the paper trail of ownership. Before you do this, you'd maybe register your screenplay, for much the same reason.

-Derek