Dedicated IP address is a need for all e-commerce websites and for those who requires ssl certificate.Without this you have to use extra steps to set up ssl certificate after all initial payments .So dedicated IP is flexible for having a ssl certificate. Private SSL are essential for your e-commerce store. In fact dedicated IP address, allow your website to be access directly through IP address instead of domain name. This is a extra advantage in term of speed of locating the website and speed of retrieving content. In large website architecture, each website will be having their own dedicated IP address. Well if you are looking at cost factor and thinking to solve this problem through shared ip then stop you are committing a big mistake as by doing this you are making your website a burden. If you need a fast website, get a dedicated IP.
Sometimes your ISP may block your if for some reasons and this might cost your business. A blocked IP cannot proceed with its business. So You will need to find a dedicated IP address, and you will free from the blocking issue. When there is a need, you will need to look for a dedicated IP address.