For myself, I only buy full towers.
Air flow, extra working space, future part compatibility, the list goes on.
For others, e.g. when I put together a build for my grandparents, I'll just use a midi tower or something along the same lines. They are never going to upgrade, never going to need great airflow, never going to need to work on the pc, etc.
So I guess, as Baez said, it depends on the user.
For people like me it's a necessity.
For the average user? I'd say overkill.
For the average gamer? I'd always recommend it.