Open Source gives the Ctrl; you can see what is going on through the source. You are free to modify and extend it. You can make it better to make your solution better. Given sufficient effort (and time), it is mailable.
Open Standards give the Esc; once due care is given to the use of standard apis, the escape hatch is always open. If a better implementation comes along you are free to go, you are free to switch-out an implementation.
The combination of open source and open standards is a perfect match. While the proprietary extensions will still exist, the good extensions are eventually subsumed or become de facto standards in themselves.
Access to the source provides the ultimate freedom to protect one's investment and influence the future direction of a project. Patch the source and if you get it right, there is a good chance that the patch will filter through to the user community. It may even eventually make its way to the standard.
Note: With FOSS, there is also intrinsic Esc; at least in the early stages. The barrier to entry is so low, the barrier to exit is no more than the cost of the experience. We are all entitled to change our opinions as we learn.