I understand what you mean, and I agree to a point, but I'd put money on that probably 95% (or more!) of Metallica's fans had never heard the Budgie version, or the original version of Diamond Head's "The Prince" for example. Exactly in the same way that 95% of the people that dug Mayer Hawthorne's version of "Maybe So, Maybe No" hadn't heard of The New Holidays. It's a way of paying tribute to your heroes or alerting people to good songs. From a "rock" perspective I've always loved when my favourite bands have recorded some covers as b-sides etc. If nothing else, it gives you a glimpse of their own record collections. And when it comes to a gigantic group like Metallica, the sheer impact of them recording a cover of an obscure NWOBHM tune is immense.