I don't believe that we need mountain bike trails "everywhere" and especially in places like World Heritage areas and the like. Yes, there is an impact to the natural ecology when a trail gets built and used.
The major trail networks in Tassie have been built in areas already affected by mining, logging, and have other users like dirt bikes and 4WDs. None of them are in "pristine wilderness"
I've heard people complaining about the Queenstown Mt Owen trails ruining the beautiful mountain, the same mountain which has been stripped bare of vegetation due to over a hundred years of sulfur emissions from mining.
We do need to be careful to not support developments in actual pristine or high risk areas (for want of a better description) as this can harm projects that are in suitable places.
I do get annoyed by people like Christine, who is a bushwalker who seem to think that all the walking tracks and the people that walk them make no impact whatsoever and use images of tire tracks to show how bad bikes are. I can also show some pretty shocking walking tracks in Tassie that are massive erosion nightmares.