This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
Also known as, "It's only a 500KB/sec. tape drive, why do I care if the burst rate is only 2 MB/sec.?" or gets good marks for "plays well with others". Most of this is relevant to all shared busses, not just SCSI. burst v. sustained performance, disconnect, command overhead, etc.