I wouldn't say it was an excuse for senseless violence. I think it showed what violence would really be like, and showed how horrible it is in some countries. Too often they don't show what guns really do to people, what terrible damage they can do and this movie certainly showed that. Plus if Rambo is senseless violence, then every horror movie ever made is too, and I love horror movies.
I'm also not recommending it to everyone, it's one of the most graphic things I've seen. I just don't think it's fair that it's called senseless.