My definition of GIS is...
Using a map to answer a question.
When an understanding of the system has been developed that allows the use of the data and interface provided, to a point that users can answer questions and relate locational information to everyday tasks the system has become usable.
GIS "IS" to each user what best answers a locational problem for them. (It could be that if a business is paying for the system then it would "need" to be a business problem). However there are many non-business problems that can be answered that could be considered perks or job benefits. These only add to the understanding and use of locational information (GIS) in the general public. In other words IMHO limiting the use of the information only limits the understanding of GIS.
Part of the understanding and use is that there needs to be a buy-in or time investment from some segment of the users that collects, updates, or otherwise changes data to add either accuracy, additional information, or currency to complete the cycle of use.
The accuracy and currency of the data can take priority over the changes and usefullness of the interface only for a short time.
The changes and enhancements to the interface need to take into account segments of users and possibly even be developed for small sets of users and groups. While making access constraints and data availablity to user groups a priority.