I think it is a great idea, as people may extend their visits to other countries. However, it is already quite easy for foreigners, especially from developed countries to visit ASEAN, so it would probably be only interesting for people on longer trips.
The problem remains only with the countries that still require visas for short stays, such as Laos, as Vietnam or Indonesia have recently removed visas for visitors from many countries for 15 days or more.
It would be beneficial for longer trips and richer tourists… and tourism professionals! 😉