Update, May 18, 2012: Donna Summer died at the age of 63 on May 17, 2012. Please see "Dance Clubs in Heaven Are Playing Donna Summer Songs."
Some of her songs are silly love songs, as are many great artists' songs, but always sung with vocal perfection. Some of Summer's songs, however, did have gravitas, and plenty of it.
Why hasn't Donna Summer been given the same respect by the public as Whitney Houston? I don't know for sure, but my hunch is that Donna was dismissed by many because she was an icon of disco in its early days, when it was too convenient to mock. Donna laughed all the way to bank, of course.
The public be damned, however, Summers is highly respected by her peers in the music industry. In fact, she is a 2012 nominee for induction into the Rock and Roll Hall of Fame.
If her knock-out performances of MacArthur Park don't convince you, listen to Donna Summer sing "Don't Cry For Me Argentina" live with only a piano behind her, and no studio trickery.
Post Script: Thanks to the following for their mentions of, and links to, this post: