I reckon it would be real smart for them to include PS2 games on PS+ once they get them running on PS4, ideally in addition to the games we already get.
People would like it and it might help drive subscriptions, but Sony could also offer PS+ slots (and the associated payment) to publishers as an incentive to jump through the hoops required to sell older games digitally. I can imagine there would be many cases where a publisher might have cold feet over doing the work to rerelease old stuff if they had no idea how much revenue it might generate, and Sony could mitigate this by effectively guaranteeing a certain amount of sales upfront. Could be a nice carrot to dangle publishers who own the rights to games with limited/niche appeal.