Some people who want to register guns don't want them taken away. Others do. Is this any different than any other political topic ever discussed anywhere? Any information our government collects or position it puts itself in, even if that was a smart move, will likely result in it positioning itself to do something evil and/or stupid in ways it couldn't have before. The development of a military is probably very high on that list, but that's one of the very first things we ask our government to do.
If variable speed limits are a good idea, certainly the government is now more flexible to lower speed limits to asininely stupid levels than if they're painted on 60 mph, thereby decimating the economy.
If having a nuclear arsenal is strategically smart for the US military, certainly it gives them far more flexibility to kill massive amounts of innocent people.
If making people register their vehicles works for efficient tracking of property, certainly it gives our government far more capability to wreak havok on us with onerous taxes if our vehicles aren't fuel efficient enough (or American enough

).
There's far, far worse things that our government could do with the information and tools it has now than what it could do if it knew who owned what guns. I'm not saying we become lazy thinkers, but just that we don't throw out good ideas because of "slippery slope" arguments that would much more appropriately apply to massive amounts of things the government does without taking it to the most tyrannical conclusion possible.
For the most part, I've gotten to the point where I want to judge our government's actions mostly on the face of the quality of the idea itself, and not on some feeling about what
could happen, since if I start thinking like that I already live in a horribly scary, scary country.
"Men did not make the earth. It is the value of the improvements only, and not the earth itself, that is individual property. Every proprietor owes to the community a ground rent for the land which he holds."
- Thomas Paine