what to open with when all depends on a ton of things going on around you. the speed of the game, your table image, and your goal in opening are all extremely important. i didnt see the specific hand in question with the 5T utg raise, but chances are the guy was simply playing the hand to make a play based off his table image and pick up a small pot on the flop. risky play but with a tight image, good players will see the raise as a sign of strength and fold their hands to a good continuation bet on the flop. the question of if you should only play the top 15 (or whatever) hands really depends