Fex are T2, right, but Nids need that bulky tank to stand against the ranged weapons of marines. They force marines to focus him instead of his fragile brothers, and that makes up for the power level of the nid T2 classes and the lack of proper ranged combat.
You do realize Marines are at a huge disadvantage in cc ranges? For instance, the blue console in Hive is very hard for a Marine team to cap, because their are 3-4 blind corners and 2 places where nids can drop right on their heads. A terminator in front, with his tankieness, forces the nids to attack him while the other marines focus their fire on them, much as a Carnifex forces marines to fire at him ignoring the Tyranid Warriors. It is the same idea.
In short, if Tyranid gets a class which allows them to semi nullify the range advantages of the Marines, ie the Carnifex, then it is perfectly reasonable for marines to also have a tank class which stalls Tyranids and allows them to remain in ranged combat longer. If Tyranids stand a chance moving in from long range, Space Marines get a chance in short cc ranges.