Defending Minas Tirith a strategic error?

[Son of Númenor]

In light of its outcome, I don't see how the defense of Minas Tirith could be construed as a strategic error.

However, for the purpose of the thread, I suppose we are putting aside the outcome to look at what other possibilites Gandalf, the Gondorians and the Rohirrim had in their fight against Sauron.

gradually withdraw in a slow, fighting, scorched-earth type pullback

Keep in mind, when the Rohirrim finally reached Minas Tirith, it was already under siege. How then, could they have engaged the enemy in another part of Gondor? They would have nothing to withdraw to except hordes of enemies between them and Minas Tirith. As for the Gondorians, they had no choice but to defend their keep: they were extremely out-manned and had no hope in any kind of major offensive maneuvre against a much larger and stronger army. Their only hope lay in defending a city that was well-protected and hard for enemy forces to penetrate.

The Rohirrim would have had no hope of fighting Sauron's foes head-on, even in a 'scorched-earth' battle; the primary purpose of the scorched-earth tactics use by the Russians in WWII was to destroy supplies so the Germans would freeze or starve to death. In the case of Sauron's armies, they were probably very well-equipped to begin with as far as food and supplies, the weather wasn't nearly as cold as the western USSR's, and the period of time in which the Rohirrim fought Sauron would have been too small for a scorched-earth policy to have any real effect on Sauron's army. The Rohirrim, instead, were able to utilize their strength. The strength of Minas Tirith was in defense, as it had been their primary means of warfare for hundreds of years. The Rohirrim were cavaliers, and so were best utilized in this battle as a charging force, who could do serious damage by attacking the enemy's weak flank, and who could draw attention away from the siege.

Gandalf's plan, even if it could be called a tactical risk (which I would dispute based on the above, and also because any military strategy they had adopted would necessarily be risky against an army as powerful as Sauron's), I don't think it could be called a tactical error. It was well-executed with the means available, and ultimately led to a victory.