Assuming Romania joins the war, having an Allied Expeditionary Force (they probably wouldn't be able to deploy more than two understrength divisions, probably drawn from Egypt and Syria) in southern Poland results in a couple of things:I don't think it's going to float, and if it does, it may very well be worse than OTL, in the sense that that small force might well be chewed up. However small that is, it would still be better used elsewhere, not to mention the reinforcement of the notion of the German invincibility.
1. It complicates the Soviet response. They might skip attacking the southern sector entirely
2. It ensures the 120k Polish troops that evacuated OTL keep fighting
3. It potentially ensures more Poles manage to retreat in good order, such as those that were captured here or here
4. It forces the Germans to continue fighting towards the south, with extending supply lines towards an increasingly narrow funnel
5. It brings the Romanian army into the fight
I don't see how that's a downside.