Crobat can be built defensively and can do it well, only because of its resists. We just really need a new Poison/Flying that is tankish in stats.
Crobat is like a Dugtrio statwise. In today's metagame, Ground type Pokemon make good walls, like Donphan, Hippowdon, Swampert, and Gastrodon for example because they resist Rock. Dugtrio is not recommended to be a wall... just because of Rock resist.
Poison/Flying is a good defensive type, but Crobat isn't suited to be doing such kinds of walling. Besides, Roost makes it 2x weak to Ground, 2x resistant to Fighting and 2x resistant to Grass, and neutral to Rock, killing off its walling after a single Roost, IIRC.
We just need to get better suited Pokemon with better walling stats so they can start walling well. Poison/Flying is a very good defensive type. Being immune and 4x resistant to 2 of the 3 most common mainly Physical type attacks (Fighting, Rock, and Ground are the biggest Physical threats) is something good to have. We just need a defensive Poison/Flying. Crobat just can't handle many Stone Edges. It's just a sitting duck unless it Whirlwinds, but then what would it do to Cradily? (Suction Cups + Stone Edge)
A Poison/Flying with Poison Heal would be an incredibly sexy Breloom counter and wall. 1/8th heal every turn, status resistant (Breloom can't status it to Sleep), and it has 4x Fight and Grass resist.