Benefits of exposing UI mockups early in requirement cycle

During a recent use case walk through session, I presented the UI mockup before we finished going through the whole use case. We ended up spending the whole session discussing the UI because people were so adamant on what they want on the UI they lost the big picture of what the use case is trying to accomplish. That said, my lesson learned is to walk through an entire use case first before showing any UI.
This doesn't mean the approach of exposing UI early in requirement cycle is inappropriate, in fact, I preach religiously the benefit of having UI as supporting document to the use case.
1. UIs are so easy and quick to understand, they accelerate the requirement cycle. They help you to expose missing requirements or reach agreement with stakeholders early, thus save time and money on having to rework the requirements later. Personally, I'd rather find out any conflicts, issues early so I can form a plan to address them.
2. Stakeholders are involved early in design and help to identify important workflows
3. UIs force the use of official or customer facing terminologies, which helps to confirm the correct use of them.
4. UIs help to improve stakeholder relations. Business users don't care much about system behavior or what happens in the backend as opposed to the shiny UI; they are reluntant to sign off on text only requirements.
5. Lastly, if we engage both sides of the brain, meaning express a concept in both text and pictorial form, we have a much higher success rate of getting the message across.

I am a strong believer in exposing UI mockups early in requirement cycle. The trick is to expose them on the bases that people get the big picture of the business goal; also the expectation that the final design may be different need to be set.