MBPS and IOPS. The more spindles you have the more throughput and IO you have. Also MDF is random IO and transaction logs are sequential writes. Isolating these read and write patterns enables the heads on the disk to work more efficiently. I have a blog post about IOPS and throughput [here](
http://www.timradney.com/iops). Disadvantage would be ummm...........
In all down to contention. A single drive cannot write to two location at the same time while two or more can. There will still be contention in the drive controller so you could have two of these also and so on. Less contention means higher read and write speeds and hence better performance. Can't immediately think of a disadvantage apart from you only need to buy one drive.