How effective were sieges as a defensive tactic? They don’t seem to be used to much effect in the books.

Usually, incredibly effective – hence why kings and lords spent so much money on building them. And even in ASOIAF, we have examples of successful defenses – the Bloody Gate holding off Halleck Hoare, Tytos Blackwood holding Riverrun against Jaime until Robb can relieve it, Tyrion holding King’s Landing at the Battle of Blackwater, the Night’s Watch in the Siege of Castle Black. 

Successful siege defenses are just not a very dramatic form of war – hence the Battle of Helm’s Deep revolving around heroic cavalry charges rather than waiting until Saruman’s army ran out of supplies and was hit by the plague.