I find it funny when people get angry that the clubs aren't taking the Nines seriously. It's a novelty event, it's not meant to be taken seriously!
It's also pretty obvious that the NRL is trying to set this up as something of an entry level product as well, something that isn't too serious or hard to get into, and that isn't too time consuming and expensive to watch, but can also act as a good gate way into following the NRL proper. So considering that we (hardcore rusted on fans) obviously aren't the target audience for the Nines either.
So taking all of that into account who really cares if legends that haven't played in a decade or some random ex-AFL player get a run, if anything having them play will attract interest from people who wouldn't have any interest in watching otherwise, which you'd hope would lead to some of them liking what they see and becoming fans.
Imagine the impact it'd have in NZ if the NRL could get e.g. Richie Mccaw to play 10 minutes in a game for the Warriors, it'd be a huge news story over there and it'd expose the game to a ton of eyes that wouldn't normally see it, so if it was on the table then why not?