Need to rephrase a bit this part:

"You must configure a default storage class for your cluster. Otherwise, the cluster cannot receive automated boot source updates."

From the feedback we've got, this can give an impression that it's the only repercussions of not having a default storage class.

So we can either remove this sentence, and only keep the 
"You must configure a default storage class for your cluster." 

or add more details, like:
The cluster will not be able to import the boot source images, DataVolumes and PVCs without storage class specified will not be able to import. VMs with Preference without storage class defined will have Pending DVs. 

    4.17, 4.16, 4.15, 4.14