If other people want to pay and be paid, that's fine, but for a lot of people in the open source sphere it leaves a bit of a bad taste.
Basically it becomes something very different once it's a product you're selling. I'm honestly not sure it's a good advice - your users will rightfully expect a lot more from you if they pay for the product, and you'll probably not make enough money for it to really make sense. So it'll be more work, more obligations, and monetary incentives won't be strong enough for it to make sense.
Encouraging users to make donations to cover the cost of operation, on the other hand, makes all the sense in the world.