It depends entirely on the nature of the integrations. Don’t think of Integration Server as “B2B”. While the focus is on XML, http, ftp, etc. these technologies are useful not only for connecting to external entities but can also be applied successfully to what has in the past been referred to as the A2A environment. Exchanging XML documents over standard protocols is fast becoming the defacto approach for any type of integration (web services anyone?).
There are times when Enterprise Server is more appropriate than Integration Server. From my narrow view of the world, these times are shrinking in number. The primary advantage of ES at this point is the straight forward publish/subscribe facilities that it provides (some will argue speed of transport is another primary advantage).
If pub/sub isn’t important for your integration–and most times it is not–then you don’t need ES. Pub/sub is important when the recipients of a message are 1) many and varied; 2) changing. Most business integrations are essentially point to point. (Do you need to “broadcast” a purchase order? Probably not.) IS is a document broker that provides sufficient decoupling of applications and can be configured to provide crude pub/sub capability if needed. And it is FAR, FAR better at handling XML than ES ever dreamed of doing.
Of course that’s just my opinion. I could be wrong.