From O'Reilly PC Hardware in a Nutshell 3rd Edition:

Unlike PATA, which permits connecting two devices to a single interface, SATA dedicates an interface to each device. This helps performance in three ways. First, each SATA device has a full 150 MB/s of bandwidth available to it. Although current drives are not bandwidth-constrained by PATA interfaces, as faster drives become available this will become an issue. Second, PATA allows only one device to use the channel at a time, which means a device may have to wait its turn before writing or reading data on a PATA channel. SATA devices can write or read at any time, without consideration for other devices. Third, if two devices are installed on a PATA channel, that channel always operates at the speed of the slower device. For example, installing a UDMA-6 hard drive and a UDMA-2 optical drive on the same channel means the hard drive must operate at UDMA-2. SATA devices always communicate at the highest data rate supported by the device and interface.