If you are collecting sensitive information from site visitors (e.g. credit card information), or running an online store, then having a certificate is absolutely necessary. The only exceptions to this would be when you use a third-party provider e.g. Paypal, to provide all the necessary security.
However, these days even collecting personal data such as name, address, and telephone can be considered sensitve information and having a SSL Certificate is usually recommended if you collect that information. If you do not provide a secure connection when collecting this information people are MUCH less likely to provide it to you.