Why wouldn't they use them? You seem to have some warped sense of your Space Marine Chapter. First and foremost they ARE a chapter that follows the Codex Astartes laid down by the Ultramarines Primarch. That means that they will follow the rules laid down by chapter structure, seniority, equipment, etc..
Just because someone is better at one thing doesn't mean that the worse in another area, especially when dealing with marines. For example, the Raven Guard deal more with jump troops and close combat, but I guarantee you they have a few Land Raiders, Predators, etc laying around. There are a few chapters who have almost been wiped from existence, but they will still have something as basic as Rhinos and Predators and such. They may only have 3 or 4 for a whole chapter because they are small and not truly a full compliment, but they will always have some terminator armor, tanks, and special weapons.
I will read the IA book tomorrow on your chapter to get a little more of a idea who they are, but if there is a option in the Codex you should not feel bad taking one fluff wise. Just dont roll out with 3 Landraiders and 3 squads of termies, 2 squads of assault marines with all the bells and whistles if fluff is what you are trying for. Other than that you should be fine
