Even with no cards, little user base, Linux keeps growing in the Digital content creation area, just look Autodesk releasing some of its big tools in this OS, Maya, Softimge (Avid), plus all the incredible open source already available.. all suporting Linux, Decklink cards apeal to lot more people , not only editors.
Also looking at the linux cow forum wont give you a clear idea of nothing..some veeery popular apps have small forums here..
Esteban