I have scene pages where when you leave them blank – the ranking improves. Not always though – keyword meta IMO does nothing but cause extra work.
I even think its possibly better to leave the description blank – as google will fill that in based on your content. A good way to tell what google thinks you content is about.
The only one which I feel has any value is title tag – and I often feel if you over stuff that one it will trip panda/penguin filters.
I guess its good practice to apply description as well – but do not think its a rank factor.
I have debated building a site with absolutely no keyword or description meta tags to see what happens.. LOL