Need was only a minor consideration in this proposal, because most are hypothetical.  In the event Bitcoin (or any other) currency/blockchain falls out of favor (or runs into a technical issue of some unforeseen sort), a mechanism such as this would allow for portability of assets to a more robust blockchain and not keep those assets irrevecably bound to a single blockchain.  In the event a liquid market did not exist to exchange MSC between itself and another cryptocurrency, a method of this sort would allow for MSC to exist on that currency’s blockchain for immediate exchange between the two (trading MSC for Feathercoins would be immediate if those MSC lived on the Feathercoin blockchain, for example).

For a "why" that lead to this proposal, for example, could a method be devised that allowed for “dividend” payments to Mastercoin holders that did not require further issuance of currency, and if so, from where would the value necessary derive?  In this case, the import/export method has value (assuming the needs such as presented above are true), and providing such a value in the Mastercoin protocol could feasibly increase the inherent value of Mastercoins for the holders.

The OP implemenation example is just a simple example to show that the underlying idea can be roughly implemented, but that refinement would be necessary to make the solution a proper elegant one.