There is a double whammy if your site goes down.
Not only are you loosing potential sales leads as people cant visit your site, but also you run the risk of Google de-ranking your website if it has an extended outage (ie greater than 48 hours.
Here’s what Matt Cuts from Google has to say about that:
Its not until you begin monitoring a website’s availability to you start to realise the amount of time a website is off-line, and this is one of the reasons we have introduced Premium Website Hosting Service with high availability and high performance website hosting bundled with uptime monitoring and reporting.
We even provide specialized services for WordPress sites too!