The answer to this question is better placed in the context of a persistent deficit balance, though even a persistent surplus balance should be avoided for reasons given below.
Persistent Deficit is Non-sustainable:
A country running into a deficit balance increases its short-term indebtedness to foreigners or depletes its gold and foreign exchange reserves.
These reserves are hardly inexhaustible and it cannot afford to have a policy of depleting them on a continuous basis. Sooner or later, it has to replenish them.
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Paying for an import surplus through an addition to short-term external indebtedness may appear a harmless and even an attractive proposition. It may create the erroneous impression that excess imports are coming in free. However, it does not mean that they are never to be paid for.
A debtor country is expected to meet its payment obligations as they fall due. Its creditworthiness in international markets suffers if it fails to do so. This makes it difficult or even impossible for it to have an import surplus in future, as borrowing options will rapidly dry up.
Thus, whichever way we look at it, a country ought to be seriously concerned about controlling its deficit balance of payments when the same is persistent and sizeable.
A Persistent Surplus is also worrisome:
A surplus BOP may be alright for a limited period just as a short-term deficit can be. The country in question may use the surplus for replenishing its depleted gold and foreign exchange reserves and/or for reducing its foreign indebtedness.
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But having a persistent surplus balance of trade amounts to handing over a portion of the fruits of the country’s productive resources to foreigners free of charge.
Such a policy can be acceptable when a country wants to aid some other countries on humanitarian or political grounds. But even that policy can be justified only for a limited period and up to a limited extent.
Normally, of course, the corrective actions taken by deficit countries should suffice, but the surplus countries should also be ready to cooperate more so for their own benefit. As seen above, a long run persistent surplus B.O.P. is also not desirable.