Interface through XI or P2P - How to decide?

We are in the process of implementing XI for a client.

The client currently has about 600 point to point interfaces connecting more than 200 applications to SAP (and to each other).

We are in the process of routing these point-to-point interfaces through XI.

My concern is:

Is there a strategy or guideline to decide which of the existing interfaces should be moved through XI and which of these should continue as point-to-point?

I need inputs from the technical angle as well as the business perspective.

In case I need to pose this question in a business forum too...comments on this will be welcome.

Thanks in advance,

Prasad Kamath