Except only the demonology spec has demon form and it's not permanent. If demon form worked like flight form that would be like saying only Resto has access to flight form, and only when in tree of life form.Since warlocks get demon form now, I wouldn't complain if they got a flight form too.
You can't really make a fair comparison with warlocks. All specs of monk have access to Zen cloud, all specs of Dk have access to Winged Steed of the Ebon Blade (aka the death chicken), all specs of druid have access to flight form.