Well on pessimistic side you can think that Zimbra employees will not write OSS code..
But there are many practical reasons why the wont..
1)OSS version is a test bed for core NE functionality so it’s a better deal for zimbra dev to TEST and release there code in OSS version.
2) Zimbra principals (original owners) have been part of many startups and buyout's so I believe they have good intensions for Zimbra as company and open source community
I will go on a limb here that we as Open Source community needs to step up and start developing core Zimbra functions then only we can demand zimbra employees taking more and more interest in OSS version in the future.
till now there is not really any great deal of open source community work on core zimbra services like Ajak UI and other integrations Zimbra did by themselves.
We as Open Source community need to come together or keep our self dependent on zimbra employees to develop for us.