Uncss is extremely useful if you are using a large front-end framework like Foundation or Bootstrap because most of the time you will only be using around 10% of the css.
I recently used grunt-uncss on a Foundation 5 project and it brought the generated css file from 149kb to 6kb minimized.
app/css/app.css - file 5007 lines (4725 sloc) 149.149 kb
dist/css/app.css - file 1 lines (1 sloc) 6.383 kb
ignore: ['#ignoredselector', '.ignoredselector']