I'm with Woodduck on this one. Surely good diction is part of good technique. I hate to bring Sutherland up again as an example, but she is often cited as a singer with a perfect technique, but if that technique necessitates reducing every vowel to a vague "aw" sound, swallowing all consonants in order to achieve a legato, then how is that a perfect technique? Admittedly this wasn't always the case; in her early recordings her diction is actually quite clear, and her performances, to my ears anyway, are enlivened because of it. Later on her lack of diction becomes a serious impediment to enjoyment, for me anyway. Even in her famous recording of La Fille du Regiment, you'd be hard pressed to know what language she is singing in. Pavarotti, on the other hand, is so clear, you can hear just how bad his French is, something that also applies to Corelli in any of the French roles he sang. Their diction remains superb, even when their pronunciation of the language is not.
Take another example, say Sutherland's recording of Puccini's Senza mamma. Here she does try to sing the words, but the words cause her to have problems maintaining a pure legato line in the very first phrase Senza mamma of bimbo tu sei morta. To hear what I mean, listen to Scotto or Tebaldi, singers with very different instruments, but both of them have perfect diction and are yet able to maintain a smooth, effortless legato line. I don't think it has anything to do with them being Italian, because Callas, De Los Angeles and Caballe all manage it too.