The primary reason why most laptop vendors don't offer a hard drive in place of the optical drive is power consumption. Optical drives use basically nothing when they're sitting there idle, and since most optical drives today are used about as much as floppy drives were used in the mid 1990's (i.e., virtually never, only to install software, and even that is going away with Internet delivery of software), that means that they can spec the rest of the laptop around a lower power draw, which allows them to claim a longer battery life and allows them to use a smaller fan to cool the laptop and a smaller brick to power the laptop and allows you to actually set the laptop on your lap without scorching your lap and requiring a trip to the surgery suit for skin grafts. The large desktop replacement laptops from Dell and HP (amongst others) actually do have space for multiple hard drives, but those are more portable desktops than laptops -- you aren't going to unplug one of those and walk across the lab with it to plug into a different network to sniff for a rogue machine that's brought your installation to a crawl, they sit on a desk and stay there, with short excursions to the car and home perhaps, because they are large, bulky, hot, will last maybe 20 minutes on battery power, and have power bricks the size of concrete blocks.
Given that, if you need a lot of space in a single 2.5" form factor package to fit into, say, a current Apple Macbook Pro (where Apple very highly optimizes the power consumption and the OS really doesn't deal well with you removing the optical drive and putting in a second hard drive using a third-party bracket, it won't go to sleep correctly much of the time), the hybrid approach makes a lot of sense. The primary issue with the prior generation of the Momentus XT in that application was a) it topped off at 500GB, and b) it consumed more power than a 750GB 7200 RPM Western Digital Black drive. The current generation solves the capacity problem, but I'll need to take a close look at power consumption, because power budgets in many modern slim (i.e., actually portable) laptops are quite tight.