In production enviroments so far I have only seen DARs with mandatory deployables, with only one exception: a database script that must always run in T and A and never in P. 
Maybe that's because the packages have been tailor made for the deployment platform, be it WLS, IIS or something else. 
The only counter example I've seen is the PetShop demo, which includes deployables for alternative platforms in one package. I would be love to see scenarios where people have done that in actual Deployit implementations as well, but I'm assuming it's rather uncommon.
In these cases I would recommend to start with ``udm.Environment.ignoreCardinalityRequirements=true`` and then turn the requirements on per environment once the packages are ok. Or set ``udm.BaseDeployable.requiredDeploymentCardinality=OPTIONAL``.