It depends. If you believe 'useful' to be achieving equality, then it could be argued that violence was a contributing factor to the success. However I believe that they legacy of such movements is of equal importance. Violence in this sense could then be seen as not so useful as questions are raised in the future related to the legitimacy of certain elements of said movements. Also, when you say violence there are several avenues to consider. Do you include riots and the destruction of property? The violence placed upon potentially peaceful protesters? Also, violence upon the self as seen in the suffragette movement?

It is possible that no movement aiming for drastic political change can achieve their goal without the use of violence, or without violence simply finding them.

Anyway, this has got me thinking now, I'll check back later with any reading I find.