A lot of Axis players do this, but I see it as a good development if I’m allies. Any left over French units could be annoying in the early rounds. You have to be sure you can counter France if they take it and get the bonus infantry.
Axis won’t get the +2. Just holding Normandy for 3 turns is 2 more infantry. Axis will need a larger force in France than otherwise to prevent Allies from getting a foothold because USA will be able land their planes in Normandy right off the bat. If you don’t contest the allied landing in Normandy, Allies will be able to trade for Southern France.
A combined Italian-German stack in Normandy is strategically preferable to a stack in France, provided you have the manpower.
If USA could concentrate all its resources on the Atlantic front it would be a different story. But dual victory conditions means that if USA invests enough to make Germany pay for taking Normandy, then they likely didn’t invest enough in the Pacific to stop Japan.