Do people really steal invention ideas?

It happens: in fact, I personally have suffered an eavesdropping industrial designer in a local, ostensibly-friendly incubator who took a concept (and even the working product name!) my partner was discussing, used it to raise money via Kickstarter, and acquired a patent for his (admittedly worse) version of the product. Of course, there is nothing legally problematic with what he did, and we went in another direction with our product, but the Busch-league industrial espionage experience was a stark reminder of the risks with working in communal shops. As already mentioned, it is most prudent to have at least a broad provisional application on file to lock in a priority date and, to the extent a receiving party is willing to execute one, a properly-drafted NDA that contractual prohibits the party from further disclosure or other use (including that which would not form the basis of an infringement claim) of disclosed materials and IP contained therein.