In my main PC where I do use an SSD as the primary drive (SanDisk Extreme 240GB), I am very tempted on replacing one of my secondary hard disks with a 2TB SSHD for a few reasons.
This 2TB hard disk has over 350,000 files, many of which are photos and documents. It takes several minutes to do a full search for files. With an SSHD, after a few searches, the sectors containing the directory records would end up in the SSHD, reducing search times to just a few seconds.
Likewise when backing up the HDD, the directory indexes containing time-stamp information will end up in the cache, so most of the time backing up will be spent backing up new/changed files than seeking all these indexes to compare with the last backup.
I also have years of e-mail archives on the hard disk and I regularly need to search old archives. With an SSHD, the indexes would end up being cached.
I did give Dataplex a test run a year ago, which is software based caching that uses a spare SSD as its cache and it sure made a very noticeable improvement with my secondary HDD. However, what made me stop using it is that I could not access the HDD outside of the OS, which meant if I had an OS crash, I would have to use data recovery software to access its content. Even still, there could still be data on the cache SSD not yet written to the HDD. With an SSHD, no software is involved, so any OS can access that drive exactly like with a regular HDD or SSD.