translation: they don't have the software
There are obvious use cases for hybrid drives at the hardware level. form factor being one, you can slap a hybrid drive into pretty much any laptop out there and get an immediate speed boost. No need go and buy a new laptop. Easy to support is another - my Linux distribution(Ubuntu 10.04 LTS) has no support for hybrid on my laptop or desktop. Not sure what the state is of hybrid on Linux in newer systems, haven't seen much talk of it.
Simplicity - no special software to manage/patch/etc.
So far all of the hybrid drives have come with very small SSDs, so the only obvious advantage I see to splitting it is being able to have a much larger cache.
It seems Hitachi lacks the software/firmware tech to do hybrid so they are talking the approach down, when it is clearly a winning solution in many cases(I'd wager the majority of cases).
I've used Seagate Momentus XT for probably two years now and it provides an enormous speed boost on both laptop(8GB ram) and desktop(16GB ram). I have seen people say the hybrids don't do anything over memory cache, well my experience says otherwise. If firefox is cached in the hybrid I can start it in less than a second, if it is cached in memory(start it, shut it down, start it again) it still takes several seconds.
The only downsides I have to them are the SSD cache is small, and when dual booting(though rare for me) it wrecks havoc with the cache, which has to get warmed up again. No worse than memory cache which has to get warmed up regardless.