They cannot be used in the MBP.
Or any laptop that doesn't have an available PCIe slot. Ultrabooks are the current target due to their compact size and their use of standard-based/non-proprietary hardware, but not all Ultrabooks will have PCIe-based storage.
Perhaps the confusion is that people think and SSD is an SSD, not realizing that there are multiple different interfaces and form factors.
Or any laptop that doesn't have an available PCIe slot.
Well technically there is the PCIe slot the Airport/Bluetooth card sits in. But the long blade drive won't sit there, and you have better use for WiFi than a little faster SSD.