In terms of pure SEO Meta Descriptions will have no effect on your rankings. But they are potentially your first chance of presenting your content to users convincing them to come to your site, so in that respect they are important. Search engines will use page content as a meta description if there isn't one and that can be fine, but it isn't always what you want.
Titles are a little more tricky, if you have keywords beyond those 70 characters they are unlikely to be "read" by the spiders, and that is less than ideal, so it's important to try to say what you need within that 70 character limit.