For a lot of B2B web applications aimed at serving "bigger" companies, customer adoption can still hinge on scotch-sipping and rounds of golf more than well-placed sign-up buttons or great viral marketing campaigns. It's been argued of late that maybe this part of the industry is becoming more "web-like"[1], but in any case it's not very web-like yet, when it comes to sealing deals.
Many times the companies who were convinced by your talented sales team to buy your product eventually demand that the product be tailored to their (sometimes very company-specific) needs. If you want to keep the customer happy, or maybe just keep the customer, you have to keep customizing.
One could make the argument that the most talented sales teams are the ones that sign deals with customers who are willing to "stay on core", but over time as your customers' own businesses evolve, the challenge grows for everyone involved.
[1] http://techcrunch.com/2010/07/25/enterprise-software-is-sexy...