Before the Black Gate

[Bêthberry]

Ah! A very good question, Mithadan.

. It seems to me that allowing his front yard to be so unkempt is a gross error in strategy that runs contrary to all military reasoning.

But are the generals always right? And are they always employing the kinds of strategy which the conditions of war require?

I have a very limited understanding of WWI tactics, but I do seem to recall that in hindsight it became very obvious (if not at the time to the men in the trenches) that the generals' understanding of tactics had not kept up with the technology of modern warfare. They used strategies and tactics more suited to pre-machine gun warfare. The very idea of trench warfare, which demanded that men rise up out of trenches and charge straight into guns with large rounds of ammunition in fact derived from the old idea of pitched battle, where one simply form lines (or regimental squares, etc) and marched straight into the enemies' lines, with bayonets/ axes/ swords, etc swinging. This was one thing when the enemy had a weapon with limited capacity to kill at a distance. It was quite another with the array of modern weapons which enabled one soldier to mow down many attackers with one weapon.

In short, the generals who learned their warfare on the playing fields of Eton lacked imagination. Perhaps this is Tolkien's way to suggest that so also does Sauron. Ironic, of course.

On the other hand, it could also suggest Sauron's lack of appreciation for the effects of technology on the landscape. He just does not realise that his technology creates (or allows) the very conditions of his downfall. A little below your excerpt in my edition is this passage:

They had come to the desolation that lay before Mordor: the lasting monument to the dark labour of its slaves that should endure when all their purposes were made void; a land defiled, diseased beyond all healing--unless the Great Sea should enter in and wash it with oblivion. (bolding mine)

[The Barrow-Wight]

In other words, the desolation was like a work of art to Sauron, and he loved the way it looked so much that he couldn't bear to clean it up. Kind of like the criminal who takes a souvenier from the scene of a crime knowing that it might lead to his capture at a later date. It seems dumb in hindsight, but the crook never imagines someone will search his pockets.

[Sharkû]

The terrain in front of the gate only provided very little cover, barely enough for three hobbits, not enough for Aragorn's army. At a point where the Black Gate is even less possible to assail because of Sauron's strength, I doubt he'd need to bother with the threat of 10 soldiers lying in wait of his storey-high fortress. As for spies, it's hard enough to hide a whole Haradrim army anyway, the enemy might as well see them and despair, an effect to which the landscape itself can contribute.
Moreover, the obstacles there seems to be at least somewhat recently formed, or at least that's possible with ash dunes, rocks (from eruptions?), and shallow pools. Such a frontyard would be hard to maintain, but maybe Sauron did do so at a time when he perceived himself as more vulnerable.

[Thulorongil]

I agree, Sharkû. All the igneous activity, landslides, and whatever else might logically have been going on in the area would make the landscape almost constantly change. Why spend time constantly clearing up over there when Sauron could be training his armies, building new wings onto his fortress, planning attacks, or gloating? Especially when there was almost no chance at all in his mind of someone using the area to attack him? Also, why spend time cleaning it up when he could use it, like Theron said, to even more thoroughly discourage anyone and everyone. So many things that the "bad guys" in Tolkien's writing do are based on frightening and intimidating the enemy. (And, come to think of it, frightening and intimidating their allies, too.)