It's very useful for those people whose mother language is not English. I'm Hungarian, and it takes a lot of efforts and permanent checking of my writings. Although I use Google translator, I supervise the translation and correct the mistakes. Sometimes I laugh a lot, when Google translator writes bullshit, this happens frequently, because Hungarian is very different from English.
I think I have not too much problem with proofreading and spelling, but stylistic and idioms are too hard. Could you help me if I have difficulties?