MacBooks Are Enclosed and Constrained
Sometimes it’s easy for many Mac owners to forget that MacBooks aren’t as expandable as a Mac Pro desktop. (In fact, this limitation also applies to the Apple iMac and the Mac mini, which are — same as a MacBook — highly integrated, with very little room for hardware expansion.)
Although you can hang plenty of peripherals off a modern MacBook (using USB, FireWire and Thunderbolt ports), full-size desktops are just plain easier to expand and upgrade with internal hardware. The prime examples of expansions and upgrades are the internal graphics, sound, and wireless networking cards (as well as the optical and hard drives) on a full-size Mac Pro, which on a MacBook are impossible to upgrade.
Adding a second, third or even a fourth hard drive to a Mac Pro is a fairly simple operation: The case has plenty of room, and the Mac Pro already sports the internal connectors and power cabling for all those new drives.
With a MacBook, however, you can upgrade the internal hard drive only with another high-capacity drive, and the procedure is best performed by an Apple technician. Adding a second drive to a MacBook can be done only with an external USB, FireWire, or Thunderbolt drive (which adds more stuff to carry with you . . . thereby cutting down on that mobility you prize so highly as a laptop user).
In a nutshell, those Mac power users who will want to upgrade their computers with the latest technologies in the future (such as hard-core gamers) should consider a Mac Pro desktop system first.