Many GUE divers do use dive computers. There's nothing wrong with that as long as you don't rely on the computer for anything safety critical.
Most dive computers make it difficult or impossible to do dive planning for more complex scenarios. For example, the Garmin Descent Mk2 has a Plan Dive activity profile. But it only supports square profile dives, doesn't properly handle travel gas, and doesn't generate contingency plans for loss of deco gas. So we have to do dive planning with other software that can generate tables.