Just the other day I was going through some old junk and found my 1st gen Nintendo DS, that big clunky thing. I considered dumping it since I have a DS Lite but decided to keep it around.
In general I hold on to systems if I don't have another system that can play those games. I have a WiiU but kept the Wii since it can play GameCube games. Still have PS2, N64, etc.
I also keep them around because I have this dream of bringing my kids up on video games on generation at a time. You can emulate most of the 8 and 16 bit eras easily but not the more recent ones.