My take on this comes from my view that it is really just a technicality. It is necessary, as I believe that there is no reason to keep it from anyone, and I believe that I should be able to make my own decision.
That being said, marriage is a federal thing, so I believe it should be done federally. If a trend picks up and the ACT can introduce new things, then the NT can. Tasmania is trying to pass euthanasia laws (probably fail), so this ACT thing is really, to be completely honest, a non event. Nothing will actually change in a legal sense.
And it's not evil Tony Abbott's fault. The Coalition wants a nationally consistent set of laws and this is going to cause an inconsistency. If the High Court find the case in the Coalition's favour, then the ACT's change will be invalid, and all those people who are acting like God has come to save us because of the change will be sorely disappointed to find that their marriages are invalid.
This challenge is VERY necessary to make sure that everything is consistent with existing laws. And if it found unconstitutional or what have you, then what difference has it made? We just need to get it federally, then it's done.