The flexibility of T Walls makes it able to cover up to 12 blocks wide entrances! It's vast versatility and flexibility made it's way into the meta quite easily. T Wall is the best wall placement for wide entrances (6~8 small blocks wide).
The reason they are bad is zombies initially have relatively low DPS however overtime their DPS will inevitably overwhelm the Repair Rate of a single worker.
The only good reason to use them is to stall or temporary block off some entrances. Flat walls are very versatile, they can fit almost anywhere with little to no thinking required.
