very important IMO, especially during the first years where the support might not be as great.
You could for example sell your old console and still get ~$200 for it. Not bad.
My biggest issue is however space. No space under the TV, no space for another HDMI cable in my TV, so obviously I would need to replace a console.
Also for people who swapped sides after a gen it's even more important. I didn't own a PS2, but bought a PS3 (no BC model since it was already "late" into the gen. 2009 or so...). Missed out on a lot of PS2 games that I could've borrowed from my brother or just buy in stores.