Well, right now I have all 4 bays full plus one in the optical! .
This all somewhat depends upon what the percentage free space is in your file system and the size of the files you manipulate on average. Also depends if you open the same files repetitively.
One reason to have 5 drives is to stripe wide with RAID-0. In a quest for speed, only use the outer sectors of the drives and read/write in parallel to 2,3,4 drives. That leaves numerous drives with lots of empty space. Space is not a topmost or secondary constraint.
Another reason to have 5 drives is that the percentage free space on your drives is low. You have lots of data to store. Also want speed, but cannot ignore the question of space and have a budget. ( can do all kinds of things if budget is infinite. In reality most folks have a fixed budget to work with. Once trying to satifying 1,2, or 3 constraints the same time need to work through compromises.)
For example if have two 320GB drives in a raid 0 set up holding 400-500GB of data now, you can just swamp out those two and put in two XTs. That would be an example of the second case. If had three 320GB drives in raid-0 holding 120-320GB of data that would be an example of the first.
The extreme case of only storing 120GB ("wasting" 100's of GB of space) is a good candidate to go to single SSD drive.
The problem that hybrid drives primarily address is that hard drive cache sizes have not kept up with the increase in storage. If you look back hard drive caches have gone from 512MB to 1MB ... to 32 MB . Because there is high pressure to keep hard drive prices down, the memory caches have lagged behind. In fact the percentage cached has gone backwards sometimes. A drive cache stuck at 32MB but the platter storage going from 500GB to 1TB. As the percentage drops the cache effectiveness drops. Down significantly below 1% not really going to have much of an impact except special corner cases (e.g, 32MB/500,000MB ==> 0.0064% theoretical max amount cached).
With a 4GB NAND flash cache and relatively inexpensive flash controller can give the hard drive a more effective cache. (4GB/500GB 0.8% theoretical max cached. 4GB/320GB 1.3% . 4GB/250GB 1.6%). Additionally at 4GB, the flash cache can be bigger than the file buffer cache that Mac OS X uses also or can cache with different time perspective. So, if as part of your workflow you open the same large files every day then the new cache will pay off. (e.g., some apps always open every day. or some project file open close every day for several days.) If you always open/create new large files once and then rarely circle back to them after several hours then it won't as much.
Putting these in a RAID-0 is going to be more effective when the logical volume is pulling the same data repetitively. The XT drives are faster than other 7200 drives on average, but with RAID-0 hiding some of the latency by doing things in parallel not necessarily going to get a big "bang for the buck" bump in substituting them.
