The pesimist point of view... Zeppelin are thieves and get too much credit for what they did... they're not that good of a band....
The optimist point of view... Zeppelin may have used certain inspirations to create the music that they made but they still broke ground in the world of rock and roll... as well as sonic art....
I personally... don't care if Zepp used lyrics or previously written music. I know plenty of songs that use the exact same chord progressions... does this mean they stole it on purpose... no... What if a song uses the same chord progression and arpeggoiation??? There are only so many notes on a fretboard... and only so many ways to play... some things are going to sound similar... I've written songs before then heard a song that I'd never heard before and have to completely throw out a whole music section because a riff sounded too much like it for my own taste...
I think the only people that have ever downloaded Taurus by Spirit are the ones that are curious to see how much it sounds like Led Zeppelin... perhaps Zepp did them a favor by "stealing" their music... Otherwise... who here would have never heard of Spirit... nor cared who they were... *raises hand*