There’s one problem with running your MacBook with the battery removed. MacBooks and MacBook Pros automatically reduce the processor speed by about half when the battery is removed.
http://support.apple.com/kb/HT2332 (it looks like page is removed by stupid apple)
[excerpt] Apple essentially down-clocks the CPU to prevent the system from shutting down if it happens to demand greater power than the AC adapter alone can provide: “If the battery is removed from a MacBook or MacBook Pro, the computer will automatically reduce the processor speed. This prevents the computer from shutting down if it demands more power than the A/C adapter alone can provide.”
What does Apple have to say about this? The official statement from its support document is that “It is strongly recommended that you do not use your MacBook or MacBook Pro while the battery is removed.”