For the customer with an Internet "mission critical app", being tied to a Tier 2 has it's own set of problems, which might actually be worse than being tied to a Tier 1.
The key word is "might". In fact, I would posit that a Tier 2 with multiply redundant transit to all of the Tier 1s could theoretically have better connectivity than an actual Tier 1. The Tier 2 transit provides flexibility that the transit-free Tier 1s do not have. Just my opinion. Anyway, it has been my experience that most (but not all) of the customers that want to "multihome" are _really_ wanting either: A. geographic/router redundancy. or B. easy renumbering. Geographic redundancy can be done within a single AS and IP block. They just don't know to ask it that way. (And easy renumbering will eventually be solved with v6. Eventually.) The demand for multi-homing might not be as great as suspected. John