Mukundhan commented about Udit Narayan's voice and pronunciation here.
I thought Udit Narayan's voice was a good fit to Prabhu Deva in Kaathalan. After that, I can't think of any other song where there was such a good fit. Remember paruvayillai in Run! I do not understand why north Indian singers who aren't pronouncing well should sing in Tamil. This includes Sukhvinder Singh, Sadhana Sargam (best of the lot) and Adnan Sami (well, he is not an Indian first, but that's not the point).
SPB mentioned it in an interview that he introduced Sukhvinder Singh in Telugu, and later in some other movies he didn't think that the singer did well with pronunciation. It is possible that most new male singers in Tamil and Telugu are highly influenced by SPB, Yesudoss and TMS. This probably forces music directors to look out for fresher voice. That explains why Udit Narayan and Sukhvinder Singh sing in Tamil. For the same reason, SPB and Yesudoss were used in some Hindi movies.
Gone are the days when the entire media laughed at Yesudoss for singing 'Therukkoile' instead of 'Thirukkoile'. I do remember our folks in Mumbai commenting on SPB's accent (note, not pronunciation) when singing in Hindi.
Now, we accept bad pronunciation by singers just as we accept Chennai girls' Tamil spoken with English accent. How's it in the North?