SSL does not insure your site is safe, in fact it can create a total sense of over confidence.
What SSL does however bring is encryption for what's sent to/from your site making it very difficult for someone to "snoop" and read credit cards, names, passwords, etc. Remember while it seems like we go right to a website, the data actually hops around before hand, and in the case of wifi starts out literally in the open air. SSL will not stop a site from being hacked, encrypt the data that it receives, or make up for poor practices elsewhere but it does do a lot to help with that over-the-air stage of the process.
Then of course there's the consumer piece of mind. We've been trained to believe the security lock and a badge on a website means it's all safe, go ahead. The lift of one of these buttons alone can be a few percent to existing conversion rates, sometimes more.
Having SSL is really not an option on a transactional / membership driven site from a practical or marketing perspective.