I keep everything that won't need day-one-updates to work or internet. My PS2 and Xbox 360 will work even without internet in the future. Sold my original Xbox One and PS4, because they rely to heavily on game updates and an internet connection to work.
I usually keep mine. If Gamestop does a decent trade in deal (like $100+ towards a next gen system) then I may trade in my Xbox One. If not I'll probably just end up using it as a spare Bluray player in the bedroom or something.
Have most of them along with controllers and cables in a wood cabinet in the corner of my livingroom. I gave away my Playstation to a neighbor kid because the PS2 is BC. I sold my Xbox and Xbox 360, but I have held on to everything else. PS2, Dreamcast, N64, Gamecube, PS3. I have a SNES that I picked up cheap a few years ago. It was still in the box, but used. I do not play the old stuff much. Dreamcast most recently and I broke out the Gamecube to play Metroid Prime a few years back.
I keep them. The only console I regret selling (for equivalent of 5$, to a friend of mine back then when I was in middle school) was pegazus (nes clone in poland) with ton of games. Most retarded decision i've made in my life.