As long as people can be nostalgic or curious, there will be remakes of old games. It's just good business for the game companies to continue making them. There's a whole generation of young gamers who weren't even alive when some of these games, like Ocarina of Time, originally released. A lot of them might be interested in playing this new version simply from lack of access to the original.
I'm less interested in the Ocarina remake than I could be, simply from having it on two systems already - the N64 and GameCube. That Halo remake, on the other hand, is one I'm considering getting. I never played the original, so this might be a good introduction to that series for me.
The 3DS remastering makes me even more suspicious because they are remastering a previous N64 hits just to try push the sales of the not-so successful portable console.
That's not an accurate characterization of Nintendo's actions in this case. Some of the N64 remakes (maybe all, I'm not sure if they've announced more that I missed) were announced well before the 3DS was released. The system didn't have a measure of sales success at that point. Obviously every game is part of Nintendo's strategy to sell the system, but these particular games aren't coming about because the system wasn't doing as well as might have been expected. They were planned along before that.
One thing I've also noticed with Nintendo and Zelda (and this may only apply there), is that they tend to reuse game engines. Some parts of the original Ocarina engine continued to evident up through Twilight Princess. On the DS, Spirit Tracks reused a lot of Phantom Hourglass' elements. I wouldn't be too surprised if the new Ocarina 3D's improvements also show up in a new Zelda title later.