“Is Adult Tourist Day Pass for train worth to get?”
Well, it’s a maths question...
The day pass is HKD$65, unlimited access within 24hours to most of the stations within the railway system (except Airport Express and light rail).
Consider a single ticket between Tung Chung station (the one close to the airport) and Central station, it will cost you around HKD$25; or Mong Kok station to Central station is around HKD$13.
Therefore if you want to make your Day Pass worth more than what you pay, you will need to travel A LOT, but you will probably be able to save a few bucks. Just use Octopus to go worry-free.
Or if you like the ticket as a souvenir then go ahead, because you can keep it after it expires.