It's a really a cool feature for people like me... lazy people :) My wife has made an account so we can put aside money for our nephew and nieces. She shared this account with me. And we both assigned this account as our secondary pin account. That all within 3 min.
And last week we did some early holiday shopping and without having to assign my card to that account first (yes, told you I'm lazy), we just payed with the secondary pin.
In the early days with ING I had to transfer money from one of my savings account firs of had to correct it afterwards. The horror :)