The high bandwidth does help when you move data off an array over a single cable.
Cases with the older Port Multiplier were capable of 250MB/sec (actual, 300MB/s was spec) was useful and came out when drives were 75MB/sec and you could use 3-5 drives over one channel. Just like you use to with SCSI Ultra160 or Ultra320 controllers.
SATA 6G (550MB/sec) takes it up a notch.
As I said, Barefeats did some tests of RAID using USB3, SATA3, and TB (maybe SAS SCSI also).
You can get 700MB/sec off any well designed RAID controller array.
A single SSD today on 6G interface is interesting.
There are SSD PCIe controllers with embedded SSD style with 1GB/sec and 10-20Gb/sec (much like TB). But those speeds are going to need computers with PCI Express 3.0 for all the bandwidth hungry graphics, storage, and other I/O.
Your Mac Pro with 4 x SATA ports do not get 300MBsec independent bandwidth for each, no, they share a common controller, that is less than even 1Gb, more like 750MB/s and LIMITED to max of 275MB/sec over a single SATA port. About what SSD use to offer and less than new gen of SSD.