Early Metallica without guitar solos would be unlistenable. The songs are long and montonous sometimes without enough tempo change's. Kirk's solos breaks them up perfectly. Solo's can bring you places riffs sometime can not. The type of music Metallica use to play solo's are mandatory. Kirk's solo's never sounded out of place and majority of the time they are the most interesting part in a metallica song. Lars was pretty basic and after Kill 'em All James rythmn playing was more heavy,slow,stripped making solo's,harmonies really important. Jame's riff deliver and song writing was not exactly perfect plus montonous, someone needed to break it up.
Kirks solos in Disposable Hero's are ace especially. The one in Shortest Straw sounds wicked. Kirk had many great solo's.
And Justice For All is almost ten minutes and repetitive. Take out the leads and the song is not listenable imo.