A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
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it's describ reasons to support Linux (and MacOSX too in this article) for game : http://blog.wolfire.com/2008/12/why-you-should-support-...
I found a 3D Engine with editor ane webplugin for all system (win/mac/linux) and standalone player for all this os too (and for consoles + phones...
There are more users in Linux (all distribs) than Mac... and with LinuxSupport you have a big part of Android Support (GPhone OS, that a linux OS)...
You don't have to provide distribution specific package, only a binary installer is ok (or with source is better) .Distributions themselves build...