Difference between professional and amateur illustrations and covers.

A couple of weeks ago I read a post about how awful amateur illustrations and covers of books are. So, I couldn’t refrain from posting my thoughts and opinion too.
A friend of mine who is a graduate in book-publishing once told me that in the industry it is a well-known fact that the worst illustrations were made by professional artists. At first I was surprised to hear that, but then remembered my personal experience and realized she was right. When I was a kid, I was really frustrated by the illustrations in The Hobbit for example (the Bulgarian edition I mean). Bilbo Baggings looked so mean and greedy as if he wanted to kill all the dwarves and steal Smog’s all treasure. Orcs were so skinny and disproportionate that I was wondering how they could stand on their feet, let alone fight. The dragon Smog had so small wings that compared to them even T. Rex’s rudimental paws looked big… Almost all the illustrations had very little in common with the story! They looked that way as if the illustrator took the characters from another book and used them in J.R. Tolkien’s story… Though the illustrator was a professional artist…
I guess many of the other readers and writers could share such examples too? And many of them would agree with my opinion that for some of my books many authors must use their own illustrations? I guess if I hired a professional artist for some of my works then probably my Mountaineers would be clean-shaven, the mermaids’ hair would get wet all the time, the Brown faces would look like the Afro-Americans of today (and all the readers and critics would accuse me of being racist), etc. Of course, every rule has its exceptions, there are amateur illustrations and covers that look awful. But I’m certain all the readers and critics, writers have similar experience like mine with many professional artists’ works?
http://www.strategicpublishinggroup.com/title/TaleOfTheRockPieces.html, http://allanbard.hpage.com, http://allanbard.hit.bg,
http://www.youtube.com/watch?v=0yw3a5n00FI,
http://www.shelfari.com/allanbard, myspace.com/ivan_allanbard, http://www.facebook.com/ivanstoikov.allanbard