I can think of one good reason:
Not everyone you might want to peer with on the West Coast is at the PAIX.
Curt-
While that is certainly true of PAIX, it is also true of MAE-West and PacBell. Not everyone is everywhere. On top of that, the "quality" of peering with a given peer may vary from exchange point to exchange point, depending on how that peer is built out to each of the exchange points in question - a factor that is difficult for the exchange point operator to control (or perhaps even know). Anyone considering an exchange point should do so in a well-informed fashion. Bill Manning conducted an "Exchange Point Engineering" BOF at the S.F. NANOG, the notes for which are at: http://www.academ.com/nanog/feb1997/exchange-point.html A lot of what it takes to be well-informed can be derived from that. Stephen