They would be encouraged, but not obligated, to contribute further refinements back for the other contributers to use, possibly to be placed in the main code base. Taken a step further, if the code base was opened for anyone to use then the system would likely be peer reviewed and stress tested by more people, and thus even more debugging and possibly additional development would occur. In fact, even if some users never contributed anything back the group hasn't lost anything, it just hasn't gained. Even more important, this means that all generated business data is always in a documented file-format and thus can easily be exported. Lastly, with others using the same code elsewhere, there's a good chance of support groups emerging on e-mail lists or newsgroups to give advice to new users and for unusual situations.
Users can help each other rather than always wasting the programmer's time.
Unlike most commercial software licenses which protect the software producer while limiting the consumer, OSS licenses protect the consumer's perpetual rights to use as many copies of the code as they want at no cost. The consumer is allowed to change the code to suit their own needs, and to give those changes away to others if they wish. Will never fly. This truth is it is a nice idea, and it does work. For over 20 years individuals and groups around the world have been working on different OSS projects.