Alabasta is a core story that set up Luffy as an very potential threat to WG, introduced the Shichibukai system to full, showed that politics are a great power that can rival physical strength in the story (which was a main part in many other arcs), show that Logias aren't invincible, and pump Luffy's bounty up a notch, introduce the Ancient Weapons, recruit Robin, and a ton more stuff.
Conclusion: it's canon.
