Yes, Nintendo is obviously going to be re-using code that's probably only on your cartridge. That's even more true for some of the third party companies out there, even ones that haven't gone under yet.
The reason they should do it is - amazingly enough - to satisfy curiosity. I never said people would buy stuff for playability. Some people have a historical and not just financial interest in these things, as you know
I'm not one of the "omg free ROMz to play!!1" people (at least, I don't think I am); I like to see how stuff was put together and evolved over time.
As you say, DreamTR, they can use their IP in the future for a quick buck, but I don't really see how they would use some of it directly or even indirectly (we're talking SPC tunes and Mode7 graphics here, that's old even by NDS standards) without issuing a special enthusiast's historical release. The alternative is that they just sit on it forever and ever, which is certainly possible. We'll see - there's a lot more time ahead of us than behind.