i think they are some good rock artist with bad voices, Bob Dylan and Mick Jagger are some i can think of right now.
as for hip hop artist i do feel at times if you have a good voice it helps ALLOT for your records.
I am a fan of Talib Kweli, but it was really hard work to get past his voice at times. but i do agree its more to do with the voice then accent, i think an accent makes the artist more unique. but a poor voice can be a negative impact on listeners.