So that people who know what they're doing can use aircrack-ng to its full extent (without having to command-line it - I like command-line, but most prefer GUI, WPA handshake capture included.)

A GUI for a tool is supposed to do one thing - make the tool more user-friendly/convenient/intuitive to use. Meanwhile, aircrack-ng doesn't exist to make it quick and easy for people to hax somebody's wifi network. It exists to provide a convenient interface between the user and wifi networks, for the purpose of pentesting of all wlan networks - just because WPA doesn't crack easily, doesn't mean there aren't people who might want to capture WPA handshakes, and by extention, the GUI for aircrack-ng should include complete aircrack-ng functionality.