Yeah, that doesn't happen.
To start, the turret HP.
Level 18, the base on the ult turret is 1450 health ((50x18)+550) with 80 armor and 65 magic resist. The AP ratio for its health is 200% of heimers AP at level 18. Say you have 700 AP at this point in the game. You now have a 2150 health monster turret that deals 330 damage with autos (1.05 attackspeed) and 830 with the beam, slowing by 25% with autos (not including Rylais slow)
Any ADC with half a brain will NOT walk into range of an ult-turret just to kill it a bit faster, as it deals massive damage and outranges every marksman in the game (not to mention probably putting you in range of heimer himself)
Oh, and did I mention that it IS NOT ALONE?
He has his 3 "little" turrets
Each of them will have 855 HP, and deal 139 damage per auto (1.75 attackspeed) (20% rylais slow) and 385 damage with the beam (40% rylais slow).
Sure, an adc can kill those, but you should not have those near the adc except to finish them off post burst.
Now, take a moment to consider that in addition to these, he has a low cooldown AOE stun and his missile barrage dealing 891 damage with no ulti.
This is not what we call useless.