Bottom-line: Storage on a mobile device is cheap. Bandwidth is expensive and not always available, and the most strategic element of any mobile device is battery life.
Regarding the sales guy – if you were to have the latest customer or prospect information with you… such as an article announcing increased profits or declaration of bankruptcy, would that not enable you to be better prepared in interacting with the account? Or what if the sales guy knew the customer had placed an order earlier in the day, and he could augment that order with some additional items, would that not help him/her close additional business?
Say you are a field engineer, fixing customer equipment. If you have the information on your device that indicated what was and was not included in the maintenance contract and any work done outside of the contract required a customer signature, would this not enable the Engineer to be more productive without having to call back to the home office for more information or make additional trips to the customer site?
Bottom-line: Most often the data sitting in home office databases can enable a company's mobile worker to differentiate themselves from competitors, resulting in greater customer satisfaction and retention… and they can be a referral to prospects on your excellent customer driven service.