He can use them, He just being smart and conserving chakra when he has a rinnegan and an MS activated at the same time.
Obito has shown to be able to use the rinnegan fully,
Obito even Stated that he can revive Madara [who died years ago] with Rinne Tensei which is the 7th path indicating that He can use any tech he wants, It just wastes more chakra to use With one Eye instead of 2 being balanced.
That is the reason why Obito will die if he used Rinne Tensei on Madara because He only possess one eye and it takes more chakra to use a tech than someone like Nagato who had both eyes.
Also, Kamui is Better than using Preta path in most situations because why not phase through it rather than Using a tech that leaves you venerable to getting attacked while Sucking up chakra.
All in all, He can use the rinnegan, It's just Obito being Smart and conserving chakra and Knowing what tech to use at the right time.
It is also Plot why he isn't using rinnegan techs vs kakashi because it would be overkill. Sometimes, we gotta take things out of context and see what kishi is trying to do.