Would you join a company in whose purpose you don't believe?

Absolutely. Money is money, a job's a job, and honestly, day to day software engineering is pretty boring, whether you're working on ISO-9000 compliance or a Mars Rover. Yeah, writing code for a more "sexy" project makes for a better story over drinks at a cocktail party, but that in no way makes up for the fact that you're miserable for 8+ hours a day, 5+ days a week.