The time saved is equal to the amount of time save per access times the number of accesses. So YES frequency matters a lot, like it is the single most important factor.
Data that is read once might be 10 milliseconds faster from SSD and that's all the time you save is 10 milliseconds but data that is read 1,000 times per session will save you 10 seconds total. You are not likely to know what data you are frequently accessing. It might be some executable code in the /Applications folder that keeps getting mapped in. It's certainly NOT that Word document you are working on because it would be cashed in RAM and read off the disk once. Most of the stuff is going to be bits and fragments of the operating system or maybe some media files you are editing.
And then you have the problem of when you leave home you have to copy back all the changes to the data you made on the SSD to sync the HD.
I think Apple's fusion drive is the best idea for a non-removable SSD but this seems like a good way to used a small SSD. Adding a 128GB SSD might make the computer MUCH faster with little effort and then there is nothing to do when you leave home or office but unplug the drive, no re-sync needed.[...]
The thing is, not every frequently accessed file needs to be accessed at highest speeds.
There is practically (user facing) no difference between saving a 5MB Word document to HDD vs. to SSD.
Will you be able to tell the difference? Maybe, sure, but will it improve your workflow?
Is it worth a solution that involves proprietary data storage technology?
If you are like me, all the executables are on the system drive anyways, along with all the scripts that execute and files like libraries of applications like iTunes, Aperture (farewell...
) and so on.
The actual media files are on drives that need not be very fast.
How fast does that video of yours need to be streamed from your storage to RAM?
Anything above bitrate+overhead will do.
1080p plays very nicely from a USB2.0 drive, why would I like to cache it to SSD only because I may access it frequently?
Just an example, but there really is no need.
Frequency does multiply the time you save each time, but not all time saved/all maximum speed is needed.
Since I have a system for storing my files, personal files, in folders, naturally it's easy to assign drives as well, they are practically just a sort of "top level folder" in my thoughts like Music and Documents to others within their home directory.
From there on come the subfolders and the eventual filename.
So all files that I do not know of theoretically are on my system drive anyways.
Make an SSD your system drive, solve your speed problems.