Our clients often come to us to help upgrade a legacy ERP system to include e-commerce. Often the ERP system in question does not include an e-commerce component or the client does not like the tool that is provided. Recently we were asked by a client with IBM Web Sphere tying into their system to remove Web Sphere and replace it with a fully custom engine. While these projects are not trivial to complete a fully custom build is often easier then using the provided ERP solution.
Often the online system provided by the ERP vendor is a huge product requiring a significant amount of lead time to setup and develop but what our clients need is a simpler solution. Often clients with 1,000,000 SKUs only want to sell a limited selection of SKUs online. For instance we have a client who is a distributor for a national product line and they did not want to sell new products online, only a small set of replace parts. In this case using a system like Shopify, Magento, or Spree can be a great way to build a rapid prototype and integrate it with the ERP or WMS system to get our clients selling faster.