There are a fair number of collectors who will take a $100 chip home from the casino for their collection. Above that denomination, I'd say very few. It's not uncommon for serious collectors to value an obsolete $500 or $1000 chip at more than face.

Charles is right though that the majority of higher denomination obsolete chips are available well below face value because the chips were dumped on the collector market after the casino closed. In some cases, such as Dunes Las Vegas, the owners went out of business and ran no other regulated casinos so the gaming regulators had no way to enforce destruction or canceling of the chips. In other cases, insiders were able to obtain some of each chip before destruction and eventually disposed of them to dealers or collectors. In addition, some of the venues outside Nevada and New Jersey have pretty weak control over what happens with casino chips, so who knows what happens when a casino closes or is sold. Even with New Jersey strange things happen, like the surfacing of thousands of the Playboy chips buried in Mississippi years after supposedly being destroyed.

As to why collectors do these irrational things, wait a few years... you too may catch the bug that causes it all [g].