The portability clause does not obviate the need for a bypass trust:

• Portability clause only in effect for two years
• Relying on portability, rather than setting up a bypass trust, does nothing to address asset appreciation. If a trusts holds the $5 million exemption, the beneficiary receives the $5 million plus appreciation free of estate taxes.
• Relying on portability alone does not afford the creditor protection that a trust can provide
• Even if portability is extended beyond 2012, its utility could be reduced by future changes in the exemption amount or by divorce and remarriage. 

Attribution: Pitfalls Of New Estate Tax Portability, Private Wealth