Once again, why? Absolutely everything Stalin wants in the "people's democracies" from purge trials to military and economic integration of the bloc, can be (and was) accomplished without this crazy idea.
The simple answer is that the OP assumes he does. This is alternate history and we ask ourselves what happens if something is done. if the argument is that Stalin had reasons not to do something is carried to its logical extreme, then we end up with OTl is the only answer
Annexation does change things. For one it gives the Soviets far more control over the Eastern Europeans. Throughout the Cold War, the East Europeans proved troublesome for the Soviets. That they had their own armies and police made much of this possible. There's a reason that the Czechs, Hungarians, Poles and East Germans made more direct challenges to Soviet leadership than Ukrainians and Kazaks. Romania even got away with it
Nor can we just say that it was the weaknesses of the post Stalin leadership that allowed for this. After all, the same weak leadership that allowed the East European resistance is the same leadership that held the internal reins tight
While the thread refers to Eastern Europe, the advantages of direct control can best be seen in korea. There, the North Koreans were given independence from the Soviet Union and promptly brought about a war which Stalin did not want. If he had incorporated it into the Soviet Union, there would never have been a Korean War