How do opportunistic infections affect an immunocompromised or AIDS patient?
Clinical diagnosis of AIDS in a person infected with HIV is based on the following criteria: 


A CD4+ T-cell count below 200 cells/µl (or a CD4+ T-cell percentage of total lymphocytes of less than 14%)


OR


The patients has one of the few "indicatory" diseases that strongly suggest severe immunodeficiency. See the list of this diseases here. Usually they have fatal consequences.


As we can observe, there are mostly fungal and viral infections that live inside the host cells or (in case of tuberculosis) diseases caused by intracellular bacteria. They can be overcame only with working mechanisms of cellular immunity. Other mechanisms are simply too slow, not specific or not precise enough. 

Another issue is an impairment of the immunological surveillance and much higher risk of cancer in individuals diagnosed with AIDS. Precise elimination of malignant cells is fully dependent on cellular immunity. 

[EDITED]
For further reading, recent paper analysing the most common causes of death in patients with AIDS in the era of anti-retroviral therapy.