Impossible to apply general advice like this to every situation. As usual, reality is "it depends"

Here's a legitimate reason to keep your product or service stealth: when you're in customer development mode and you have plenty of customers to represent your possible target markets and therefore plenty of feedback, and therefore more feedback and exposure would be redundant and likely mean disappointed customers. Why burn through a lot of potential customers when you're not ready presenting them with a bad first time impression.