Working as an IT-Architect you should always strive to recommend the best solution for the specific problem at hand. This is often a pretty easy rule to follow but when it comes to the "You should move towards a Service Oriented Architecture"-advice I haven't had a waterproof explanation why this should be the right path to follow. All the hype and different definitions of SOA has made it a though area to give advice about without becoming a snake oil salesman.
This was until I read Setrag Khoshafian great book on Service Oriented Enterprises. Setrag identifies SOA as an important foundation when digitalizing and automating business processes which in turn is key for effective Enterprice Performance Management (EPM). All this is essential for continuous improvement of your business processes. The image to the right shows the core components of a Service Oriented Enterprise where BPM serves as a bridge between Business and IT. So when customers ask "Why should we invest in SOA?", I think the answer should be: To create the nessesary foundation which enables you to automate your Business Processes which in turn boosts your EPM efforts. I don't think you should expect to save money on these kind of IT-investments but instead it will put you in a better position for meeting the markets demand for rapid adjustments of business processes. And thereby gaining market share from your competitors.