the speaker believe the assertion that the best test of an argument is its ability to convince someone with an opposing viewpoint which is based on the reason that Only by being forced to defend an idea against the doubts and contrasting views of others does one really discover the value of that idea.
while I believe that there are probably three cruial reasons held by the speaker that idea can only be discovered by being forced to defend an idea against the doubts and contrasting views of others. firstly, the advantages of defending an idea are obvious: the idea can be examined and tested in every possible respect and thereby supported by every possible way, by which the idea can be well substantiated and recognized by both the arguing parties. For example, the heliocentric theory aroused lots of conflictions since it was totally controvercial with the dominating earthcentric theory. By the defending of both sides, the scientists of each side continued to find evidences to defend their supporting theory and trying to resist the other theory and eventially, the heliocentric theory won the battle and rewrited the history of astronomy by using scientific methods and substantiated evidence.
Next, by the process of supporting this idea, the person must find all possible defacts and omissions and then repair it or convince it, by which the content of the idea can be fully conpleted, and the speaker will realize that the possible flaws of his or her idea. in the example of the discussion of a unsettled business strategy, almost every one has his or her own reasons for holding that idea, and of couse they usually see the possible profits of the strategy not the flaws or omissions of the whole plan, so in the discussion and argument, every one needs to find ways, such as strengthen his ideas by finding supports or weaken the opposite plans by attacting the possible weakness, to better defend his own strategy. In the consequence, they will realize the flaws and the weaks of every possible plan and through comparing them the managers or executives will find a better one for the final choice.
Finally, by the argument, the idea can be proved its practical values and the moral as well as legal values. such as this kind of idea: it is unlegal and unreasonable for poor father who cannot buy his son a Christmas present to steal from a closed shop. The stands of possible conflicting  parties can be focused on the standard of morality and legal values. And after the defending of the idea, people of both side have to consider the most central question that whether the objection of the stealing can make the theft be viewed as reasonable and rational, and with this answer made by the question, other cases of the same kind can be provided with more practical and emotion-acceptable customary.
conceding the benefits of this method, I also believe that the extreme assertion that the argument is the only way to achieve the goal that realize the possible value also need further consideration. In fact, the flaws of this method are also very apparent, such as the argument may become some useless quarrels or some meaningless oppositions of stances or ideologies. Besides the method of arguing which should be considered as exceptional ways, there are other ways of achieving this goal such as by explainning the idea or analyzing the idea simply in a critical way.
consider the flaws of the reason and the standard which measures the value of an argument is also make the assertion that the best test of an argument is its ability to convince someone with an opposing viewpoint very opaque. The convincing ability of an argument is based on the reason and the logic, however, the test for the argument is probably focused on other respects, such as the moral concerns of the "theft" example mentioned above, and the practicality of the stragegy or the history value of the arguemnt. So the assertation is unsupported by the reason the speaker acclaimed.