When weeding a vegetable garden, one should not try to remove all the weeds.
Conclusion (should)
When weeding a garden, one should not try to remove all the weeds.
Evidence
Even though, fewer weeds would make a garden more productive, the slight productivity loss you get from leaving those last few weeds in pales in comparison to how annoying it is to find and remove every single one.
Evaluate
We need to ask ourselves what principle underlies this argument. The author seems to be Weighing Tradeoffs. One the one hand, the more weeds you remove, the more productive a garden you have. On the other hand, leaving a few of them doesn't hurt productivity much, and it allows you to avoid the painstaking effort of finding and pulling every single one.
The author seems to be citing a general principle of Diminishing Returns. When you compare the effort invested to the return on investment, you're getting diminishing returns by the time you hit those last few weeks, so just leave them.
Goal
Let's look for something like: Even though an A+ job is better than an A, the extra effort to get from A to A+ isn't worth it, so one should not try to get the A+.