Question is in the title.
No. there is no reason to do that. Even zygarde 10% is not in OU but RU and the ability power construct has been banned to OU.
As written here.
On the plus point zygarde 50% has 600 as its bst whereas zygarde 10% has 486 as its bst.
Sure zygarde 10% is faster but is accelgor in OU because of its speed? The answer is no.
So zygarde 50% outclasses zygarde 10% in everyway except speed and that too is made up for by its great bulk and is only 5 points slower.
Hope I Helped!