The cut function is finally implemented by standard in Lion as a result of modifying the paste command. If you mark the files with copy as usual, but instead hold down the ⌥ key in addition to the normal past command - it retroactively cuts the original files as well as placing them in the new destination.
⌘+⌥+V = paste + cut in Lion.
This makes an accidental and harder to recover scenario less likely.
The reasoning for this could be a do no harm design standpoint. Why cut something and then potentially lose or misplace it? Imagine the harm if you selected a few hundred files to copy them (or duplicate them) but inadvertently hit the X key. Instead of forcing you to realize a cat or toddler has cut a whole folder of files, the cut only happens when you paste with the option key held down to paste and then cut in one action.