Open source is open and therefore adaptable to your own requirements. In our opinions this also one of the reasons that SAP is the success story what it is today. Although SAP is of course a proprietary vendor.
A single software development where massive numbers of customers are obligated to pay their annual fee, is a comfortable situation for software vendors. This comfort is however the cause that vendor organizations shift there focus from external to internal.
Only a field of competition keeps you sharp.
There is almost no barrier to become a customer from an open source application, expressed in license costs there is even no barrier at all. Directly there is no revenue or profit on free riding customers, but indirectly you build a sales network that is powerful an on a global scale at fractional costs.
Condition is that your software satisfies customers in a certain need. If not, even your non-paying "customers" will burn you on forums and blogs.
Costs of software development are high. In the future this will further increase due to the larger number of platforms and linked applications that software is expected to work with.
Development of open source software normally takes place on a global scale. This contributes in solving the structural shortage of developers in the western part of the world. Capacity delivered by emerging markets also contributes on a social level to bring wealth in areas where it is more than welcome.
The development of open source software is not dependant on one vendor. In case delivery of services is not as required you can search the market for better alternatives.
Going open source to free yourself from vendor lock-in is an often heard argument. Partially this is true, for a critical note on this argument read the next paragraph about the disadvantages of open source.