The value of such a consolidated online identity is the reputation that you don't need to rebuild from scratch every time you enter a new community. The semi-anonymous state of todays internet while attractive for the first glance is not very usefull. In every online community the identity of its participants plays a central role - it's not possible to have a community of fully anonymous members. The value of network lays in the possible connections between it's nodes, when you divide the network into separate compartments the value drastically deminishes - and that is the state of today's internet where each community is a separate island and gaining reputation in some new forum is like an overseas journey.

A very sound argumentation you'll find in the linked paper "THE AUGMENTED SOCIAL NETWORK", I don't wan't to copy it here.

Update: A practical example of a topic that would benefit from a cross border communication is templating. This technique is used in many languages why not share the experiences?