All this was going to happen anyway. German troops in Southern France would've retreated back to Germany, in any case, otherwise, Patton's 3rd Army would've cut them off. The Germans would've left troops in Toulon and Marseilles and fought to keep those ports out of American hands, but eventually they would've fallen just like Brest did. And after the occupation of the Po Valley, the French Corps would've been transferred To France. So, its just a matter of timing. By destroying the Germans in Italy in July/August 1944, we would've been in a better position in Nov/Dec 1944. Marseilles would've been open by then, and we could have transferred large numbers of Troops to France, since there was no war in Italy. There was a reason why Alanbrook, Alexander, and others were opposed to Dragoon. It wasn't just some blind spot, they had good military reasons.. It ended any possibility of the Germans containing the Normandy invasion on the line of the Loire - Seine.
2. It brought an additional army of two corps into France, the main theatre of operations.
3. The liberation of Marseilles and Toulon was a valuable addition to allied logistics. No Op Dragoon and they would need to be captured. As an added benefit the railway north had not been subjected to air interdiction and could be repaired more quickly than the line from Cherbourg.
4. It brought a French Expeditionary Corps to France that could be expanded to an army.
And you can talk about how important Marseilles was before Antwerp was opened, but so what? All the offensives south of the Ardennes Before Feb 1945, didn't accomplish anything decisive. Patton and Devers pushed the Germans back to the Siegfried line and the upper Rhine.