The receptacle isn't as big a deal as is the load calc.
With a side-by-side installation your calcs look like this:
1500W - laundry circuit
4000W - nameplate of electric dryer
------
5500W total
With a stacked single-point installation your calcs look like this:
1500W - laundry circuit
5500W - nameplate of washer/dryer
------
7000W total
I made up the nameplates out of thin air, but you get the idea. The same thing is essentially being installed: a washer and a dryer. If the Owner bought a stackable washer/dryer with two plugs then the load would go down significantly.
And when using 220.84 to do service calcs for multifamily dwellings that IS a significant difference. 1500W/unit * 200 units * 0.23 Table 220.84 demand factor = 69000 -> 192A at 208V/3P. If that reduces the meter stack or service disconnect panel by one size, and your utility transformer is any significant distance away, you just saved the client tens of thousands of dollars. In this economy a couple of those may mean the difference between a project getting built or not getting built.
After the discussions in this thread I am of the agreement that the 20A laundry circuit is always required to be installed, but that you can assume it includes the washing machine load in the side-by-side scenario above.