The thing I sometimes find with covers is that people will swear that someone did the best version of the song. An example is As The Crow Flies. People will tell you that Sardinas version or Rory Gallagher's version is the best, yet they've never taken the tme to listen to Tony Joe White's version... not that I think that is the best version, but it can ring a little hollow when they can't make a full assessment.
The great thing about covers I've found is that they've led me to some great music, through hearing covers of say Gary Moore or Aerosmith, two acts I really liked when I started listening to music is that they led me to early British blues and then I listened to the earlier blues artists as I wanted to hear the songs that Mayall, The Yardbirds etc recorded. I do tend to find though once you hear the original you can quite often find it hard to beat, they have that distinct sound that makes them so appealing for others to play. There's something special in hearing an artist record one of their own songs and while other artists can sometimes cover it and improve it musically, often the covers lack something of the original - in blues this is usually the vocal phrasing that the originator used. I find it hard for example to hear any covers of say Muddy Waters or Howlin Wolf that was bettered. Usually artists make their versions too guitar driven and it takes away the magic of the original version. Others would state it comes down to personal taste, yet I believe it represents them looking to hear a particular sound, and by listening to only the covers they can often miss the differentials that made the songs so good in the first place.
"The recently formed Edinburgh Blues Club has identified an appetite for the personal communication between musicians and audience that the blues long ago perfected." The Herald Newspaper (Scotland)
http://www.edinburgh-blues.uk