First, to Red Storm Rising. I love that book, its really quite fantastic. And its an interesting and rather unique look at the a specific Cold War situation. However, its important to remember that the book was trying to explore a very specific scenario, namely one in which the Soviets were desperate for Oil and needed to capture more production to ensure domestic stability. Some elements of the plot, such as the invasion of the Middle East, reflected that scenario. Clancey needed reason to push the US and the USSR into fictional war, and oil seemed like a good way to do that. But it did not reflect the war aims of the 1980s era USSR.

Second, lets examine the geo-strategic importance of West Germany. As a matter of resources, the FRG was no small prize. The Ruhr River Valley is one of the largest concentrations of heavy industry in western Europe, and on both sides of the Rhine River exist the most important factories for Belgium, France, and Germany (both Germanies). Had the Red Army pushed to the Rhine, it would have captured most of Belgium, the Ruhr and Saar valleys, and threatened French industry on the other side of the River. And the Rhine was no large obstacle; after Caesar crossed it in 55bc the Rhine subsequently posed little threat to most advancing armies.

Another consideration was what would happen next if the Russians got hold of Germany. With Hamburg secure, the Russians could bypass the Skagerrak, the narrow strip of water between Denmark and Norway. That sea was critical in stopping Russian submarines and ships from entering the North sea, and with Hamburg taken the Russians could use the Kiel Canal to move shipping from the Baltic to the North Sea. That shipping could be used to menace the English coast, or even use the major port of Antwerp to launch an amphibious invasion or raid against the English coast. That was a major threat. Lets also examine the situation of France. With Belgium and Germany conquered, the French would again be forced to defend their borders against a hostile invading army. This would have been an obvious repeat of history. Three times in the last 100 years hostile armies knocked at Frances door, and three times they had blown past their border defences and inflicted disaster (or in the case of 1914, near disaster) on the French Army. Most planners were not confident that an allied force, just evicted from Germany, would do much better.

Finally, conquering Germany would have been seen as a victory for socialism and a defeat for the West. Whatever the Russian's reason, had they won a decisive victory in Germany, the path would have been clear to negotiate peace from a position of strength, or consummate the invasion by expelling the Allies from Europe for a second time. Either way, with Germany defeated most planners felt that France and England would have been easy prey for a concerted, decisive Soviet attack.

The last concern is the most human, though. The experience of the last 100 years had also shown that the German soldier was one of the best in the world, first class in the art of war. Further, given the numerical superiority of the Red Army, no one country could maintain a standing force big enough to oppose it. Even the combined contributions of NATO (sans Germany) was not enough. The Germans were thus integral to stopping the Red Army. But who could expect the Germans to fight to defend France? In the 1940s and 50s, many war planners proposed just that, a retreat to the Rhine and the abandonment of Germany. Setting aside the other geostrategic problems I just laid out, most Germans were unwilling to participate in a plan whose first step was to evacuate Germany. So, to secure their participation, NATO, and in particular the US, changed its plans to instead defend Europe on the Elbe, not the Rhine. This, along with a whole lot of other political concessions which are beyond the scope of this response, got Germany to commit itself to Europe's defense.

I think you're reversing cause and effect - the Soviets attacking the Fulda Gap wasn't because the Fulda Gap and Germany itself was an objective insomuch as the Fulda Gap was the ideal place for the Soviet Union to advance - given the composition of their military - in any war between the USSR and NATO.

It provided the ideal lowlands for an armored offensive - of which the Soviet Union had a lot of tanks and who's military doctrine was heavily centered on - to capture strategic points along the Rhine River and to strike deep into NATO lines.

Thus, even if there were no immediate Soviet strategic interests in Western Europe - say in your example of Red Storm Rising where the Soviet Union's bigger objective is Middle Eastern oil - the thought was that any war between the USSR and NATO anywhere in the world would inevitably be settled by defeating Western Europe in Europe for which the Soviet Union's strength would be an armored offensive through the Fulda Gap