bit_user :
IMO, the only place for hybrid drives is in laptops that need lots of storage. There, you get additional benefits by keeping the HDD spun down potentially most of the time.
In a desktop, I'd use SSD for the boot drive and a plain HDD for bulk data.
I can certainly see room for hybrid drives on the desktop. While it might be fine for a power user to manage what goes on their SSD and what goes on their hard drive, most users likely wouldn't want to mess with determining what things they store will benefit from the higher performance of an SSD, and what is fine to store on their hard drive. The majority of people would likely prefer the simplicity of having all their files stored on a single partition, and leave it up to the computer to determine which files are being accessed in ways that will benefit from being stored on flash memory.
And in many cases, this could be more efficient. As an example, some people have game libraries that fill multiple terabytes, and storing all that on SSDs could be needlessly expensive and probably rather wasteful. So, they end up with the bulk of their game library stored on a hard drive, and only a handful of their most played games on the SSD. But some games load data in ways that they don't even benefit much from being on an SSD, while others will benefit quite a bit. Short of performing thorough tests and shuffling games back and forth between drives, it can be hard to determine which is which. And even within a single game, some files may be accessed frequently in ways that benefit a lot from quick access times, while others will only be read infrequently in a largely sequential manner. A hybrid drive could look at these access patterns and automatically manage which files, or even parts of files, will be best suited to remain mirrored in flash memory.
Plus, a large buffer area could be used to accelerate the speed of files being saved to the drive. You could, for example, copy some video files to the drive, and they would be saved at SSD-like speeds to the flash memory, then continue to be copied over to the platters in the background when no other accesses are happening. And if the drive is powered down or otherwise loses power, it can still resume the transfer without data loss when power is restored. Additionally, those copied files could be left mirrored in the flash memory, to remain available at higher performance until the buffer area is needed for something else, at which point the drive can intelligently determine whether to remove it from the cache, or remove some other files that haven't been accessed for a while.
And again, any performance deficit that a drive technology like SMR might experience could be almost entirely avoided in the vast majority of usage scenarios, since the writes could be stored to the flash memory before getting written sequentially to the drive platters at a later time.