Michael Kruger makes some excellent points on this subject over at his blog, Canon Fodder. He notes:
But, there is a second assumption behind the “take your medicine” approach to the biblical languages. Many students assume that the study of the languages is useless if the specifics are forgotten at a later point. Indeed, this may be the biggest assumption in the mind of today’s seminary students.
This assumption, however, is profoundly mistaken. Even if a student forgets every single vocabulary word and every verb paradigm, the intensive study of the languages during seminary still plays an enormously significant role. Put simply, it helps students think textually.
Prior to learning the languages, most of us simply do not know how to think on a textual level when it comes to studying the Scripture. But after learning Greek or Hebrew (even if we forget it), we now understand grammar, syntax, logical flow, and sentence structure. Moreover, we understand the way words work, how their meaning is determined (or not determined), the importance of context, and the avoidance of certain exegetical fallacies.
These factors alone are incredibly important for proper interpretation of the text and preparation of a sermon. And they are drilled into our heads when we take the biblical languages—even if we forget them later.