EPTHaving played vs. a lot of wall-in tactics on various maps (not just Tal'Darim), there's a reason why MLG maps have a neutral supply depot to block most ramp wall-ins. There's also a reason why ALL pro-level zerg players react immediately to potential cannon and bunker rushes - because allowing the bunker/cannons to finish puts the Zerg at a HUGE disadvantage.
I think the crucial problem here is that cannon and bunker wall ins are a "cheesy" tactic, but if the initial push doesn't go so well or it's scouted too early, you can easily cancel the structures and come out with almost zero cost - Zero cost to the attacker, but a huge mineral (and psychological!) advantage.
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