Turkeyspit, good point on who really crushed Hitler's army. A megalomaniac, weilding the army Hitler had, and making the foolish choice to fight the war on two fronts was exactly what caused Hitler to lose the war.
As to why siege warfare still exists, may I add a quote?
Originally Posted by Stella Cadente
War never changes...
I like the line partly 'cause I played Fallout 3 and loved it, but also because it's true. War will not change, just evolve. Siege tactics in the 41st millenium are possible because of things like anti-air, anti-tank and anti-infantry, sure. But, let's not forget anti-orbital, anti-torpedo and anti-nuke technology, shall we?
In Storm of Iron, it is made clear how a siege could still be possible. Void Shields.
Void shields become the critical assault-stopper, forcing siege to begin where orbital bombardment, drop pods and bombers would have been deployed otherwise. Void shields allegedly can survive anything, if they have enough power and enough generators.
Of course, even if a city isn't void-shielded, if it is valuable enough, the invading commanders won't want to just level the place. Hellsreach on Armageddon is a good example of this. The Armageddon Steel Legion, Black Templars and other forces fought a hard battle against the Orks to keep the hive city. Orks wanted Hellsreach for looting purposes, and were more interested in fighting the inhabitants than simply destroying the hive. Imperial defenders were protecting Imperial Citizenry and government property, but more importantly, they were holding off the Orks, who, if able to take Hellsreach, would have had a stronger foothold in the region to launch assaults on other nearby hives.
The Death Corp of Kreig are not obsolete in the 41st millenium, because, aside from the reasons listed above, they also come from a world whose primary tactic for centuries was "nuke the other guy"... If their siege tactics could survive the war on their world, the Imperium can definitely use them.
That and it's also a game. It's a game where some people want to play siege tactics without playing WHFB. It's a game where it's fun to play battles to destroy a shield generator, or a city gate, or bypass a trench system. So, even with some decent rhetoric to back up the use of siege tactics, we don't even need it. Because it's supposed to be fun to lob artillery over a trench, and see who you hit. (Also, simply because the technology is so backwards, it's a more reliable tactic to duck into a trench, unless you're being hit by whirlwind cover-ignoring incendiary missiles...)