On the other hand, if you are presenting your own argument instead of evaluating someone else's then this shortcut is not available to you. To present a good argument, you have to do your best to insure that both requirements are met. Provided, of course, that you know how to do it. To test for the reasons-true virtue you also have to think, but in addition you often have to devote time and energy and perhaps money to research, observation, experimentation, and so on. Not because it's more important, but because it's harder to understand.
All mice have six legs, and all six-legged things are grey.
Therefore, all mice are grey. Therefore, Joe is a freshman. Even though the true-reasons requirement is not met. Ideally, of course, we'd like our arguments to meet both requirements. If you encounter an argument with pretty obviously false or very suspicious reasons, then it's a bad argument, and you needn't worry much about whether its reasons, if they were true, would lead, with either necessity or probability, to its conclusion.