Our friend Lars and his wife Kyra have a Living Trust with a bypass trust arrangement. This means that the share of the first spouse to die, is to be held in trust for the lifetime benefit of the surviving spouse. One of the benefits of this trust is that it would preserve the estate tax exemption of the first estate. There is a new conventional wisdom that bypass trusts aren't needed now that we have portability, by which the surviving spouse inherits the unused portion of the deceased spouse's exemption whether there is a trust or not.
Lars isn't buying this new thinking. If his estate goes outright to Kyra, she inherits his now $5,250,000 unused exemption to add to her own, but can lose it if she remarries. So a possible result is preserving no exemption of the first estate. If instead a $5,250,000 trust is established and grows in value to $10,000,000 during the surviving spouse's lifetime, the first estate gets that much exemption.
Lars has other reasons to keep their bypass trust plan. There is no portability of the state exemption, in Lars's State of Washington at least; so a bypass trust is necessary to preserve it. In addition (and maybe most important of all) a bypass trust can help to prevent Lars's estate from ending up with Kyra's next husband and his family, instead of with Lars and Kyra's children. Lars has seen this happen in other families and it's not pretty.
As a CPA Lars realizes there is one income tax disadvantage of a bypass trust. The assets in the trust he leaves for Kyra, won't get a step-up in income tax basis upon Kyra's later death. If they rely upon portability instead, the assets that would have been in the trust will be held by Kyra outright, and so get a step-up. Lars considers the asset protection of the bypass trust, against a later spouse or other third party, more important than this income tax disadvantage. So he and Kyra are sticking with the plan they already had.