Can't speak for everyone but it was and STILL IS a huge deal for me. That was the first reason why I bought a N3DS instead of buying a OG Nintendo DS on ebay, and why I bought a PSP on Ebay and not a PS Vita at a store.
I skipped out on most 7th Generation games (many because my addiction to WoW and CoD). So playing many of the older titles that I didn't play over the years was a huge plus for me. I was EXTREMELY disappointed when the Vita had no BC (there was games I wanted that couldn't be D/Led on PSN that I was dying to play... Life Valkyria Chronicles 3 and Mega Man Powered Up). That and I rather own a physical copy of a game anyway.
If the PS4 had PS3 BC... I would have sold my PS3 ages ago to purchase a PS4. If you ask me, I feel Microsoft and Sony didn't offer it initially because they known Devs were still making a fair amount of games for both, and they both felt they can still make some money off their old stock of 7th gen consoles. Now that ownership of both consoles are increasing and decreasing on their 7th generation consoles (plus Devs not making much games for them anymore) it then when Microsoft decides to make X1 BC. Odd coincidence... o.O