On the face of it, the request of the Gadites and the Reubenites to settle east of the Jordan River appears quite decent and worthy. But Moses raised two arguments against them:
1. How can you not participate in the battle for the western bank of the Jordan River?
2. You might cause the rest of the tribes to prefer not to cross into the Promised Land!
So they proposed to cross over as shock-troops, and Moses accepted their proposal. What they did appears to have been worthy, however, one should follow the approach of Rabbi Avigdor Nebenzahl, who held that it was not worthy. The true reason was spiritual: being distant from Shiloh and Jerusalem could lead to great spiritual decline.