There is a simple reason for why the iMac is so cheap;
Blue ray drives are more expensive, even more with with slot-load.
300-400$
Considering Apple would charge you 500$ for it, it would make their nice 2000$ top of the line iMac cost 2500$, a price which is much less attractive for the customer.
Of course as an option it's interesting, the screen is beautiful.
Personally, I don't really care for it, I really don't have time to waste watching movies and most bluerays movies are still the usual Hollywood crap anyway - not much repertory cinema movies in blue ray yet.