I've also found artillery of all types useful for both offensive and defensive wars.
For some reason, I always seem to get unlucky and share borders with the largest and most successful AI civs. Artillery, despite slowing down my columns, actually speeds up early wars against powerful AI civs by allowing me to take their cities with minimal casualties. My offensives rarely have to wait for additional units to catch up to the front.
On defense, scattered pockets of artillery moved to the front helps repel almost any computer attack.