The big downside with GStreamer is that it is linux specific, (no it does not work at all good on Windows I've tried to compile it and it's a complete nightmare to even make it run)
That will hinder development to new plattforms If anyone want's to, XBMC's DVDPlayer uses ffmpeg which is atleast 90% of what GStreamer uses but the big upside to DVDPlayer is that it's way easier to port to new plattforms, as it has less dependencies.
Also if we have one engine more we have alot more code to maintain (which by all means is good if anyone is willing to maintain it

).
So all in all, the upside with GStreamer might be some, the question is if it's worth the trouble. Although if anyone wants to code up something and maintain it I'm very sure we would accept and include it
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