Ars Technica has coverage of a study that purports to quantify the impact of backwards compatibility in new hardware:
http://arstechnica.com/gaming/news/...kward-compatibility-could-cost-sony-sales.ars
I always thought that one of the reasons why 3DS can pick up much more sales growth and success in Japan (and I expect worldwide) than the PS Vita is the fact that lots of legacy from previous Nintendo portables are carried over automatically.
Lets just take a look at how many owners the DS has. We are talking about millions of which many carried from the Game Boy and Game Boy Advance heritage. Many of these millions DS owners are probably more likely to be 3DS owners than PS Vita owners because they have a library of games that can be played also on the new portable.
The PS Vita on the other hand needs an extra process and extra cost to be able to play your old PSP games and not for all your games. It is of no extra process and extra cost only for the games you purchased digitally from PSN.