WerServices are certainly an important technology, but like any technology it should not be abused and used just because it's a buzzword.
Your questions are too general and lead to general answers like 'it depends'.
Before exposing WebServices you should consider who your potential users are, whether your services require secure access and whether traffic content should be encrypted. How many users do you expect?
WS is one of the ways to communicate between ISs but it is certainly not the only one. Depending on the type of information systems and communication requirements you can consider other protocols, such as RMI, IIOP etc. One of the important advantages of WebServices is that they are supported by a large variety of platforms, but the downside is that its performance is not the best.
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