He did so many covers in the 70'a because he simply didnt get many great new songs to do, becuase of the pubishing company deals and all that crap...
But on the other hand one thing is a cover, and one thing is to remake the song; back in the Sun Records era, all the material done at sun, were covers, but they were just covers? not at all, he reinvented the songs, and he changed music history with those "covers" Just as Mile Davis did on his "Birth Of Cool" another album full of cover versions.
The value of all those covers is the value of a guy who simply dig those songs enought to do them in his own terms, he looked for material he liked and serve his as a media of expression to do his art which was: interpreting those inside him, transform those into his own view and singing them which was his most wonderfull talent; lots of those "covers" became his own. One thing is writing songs, other thing is to perform the song, Lennon was a great song writer, but without The Bealtes, he was little less that awfull on stage, we was not a great performer at all. The Beatles it self were great at studio, but they were not so great live.
ELvis was a performer, a singer, nobody says The London Orchesta is crap cause they play Beethoven covers, I Mean duh! Nobody has ever said Miles Davis was a no good musician or John COltrane was a bad sax player cause they played a lot of standars "did lots of covers"; ELvis instrument was his voice, you get an Elvis record to hear him singing and to hear his arengments on popular songs "Standars".
On the blues tradition is more of the same, lots of great blues mans, as well as Jazz female vocalists, did more covers than originals, yet noboy says B.B. King or Ella Fitzgerald or Dinah Washington were not of much value to the Blues or Jazz tradition respectibly because of that.