With a GUI your installation script will only be as powerful as the GUI allows it to be. Further "touching" up can be done by hand, of course, but at that point why not write the whole thing from scratch?
So you do your programming using a DOS-Console and with makefiles, instead of (for expample) viaual studio or somthing?
Of course a GUI for NSIS must support script editing by hand, and of course there is no real need for a GUI when making trivial/little installers.
Imagine a installer for about 600 files and 90 folders - would you want to type all the SetOutDir/File etc commands by hand?
If you have no need for a GUI - fine, but I think that the existing of this thread proves, that other users have.