Keep in mind that Balsamiq prototypes look like rough sketches instead of "professional looking mockups"
on purpose.
When you show a customer a mockup that is "just a sketch" (or looks like one) you can get their feedback on the layout and content.
When you give them a realistic looking mockup, the conversation can get bogged down in minutea like in "Can you make the background less blue? Maybe like Bob's shirt?" or "I don't like that font, can you...?" At which point you have to explain that it's only a mockup, not the actual product and politely try to steer them back to what you asked them in the first place.
Also, when they see a what looks like a finished product they think the work is mostly done which sets unreasonable expectations.
