Something that is overlooked is that this model may allow you to "rent" ad2 for a month at a time.  If you end your subscription and restart it two months later, presumably you are starting from square one, so may be able to choose three new adpaks.   Or you could open another account and "rent" the use of three different adpaks.   
 
If the XLN producer pak is delivered like it was in X3, then you can use them in other seqencers.  Just include them in your VSTi path.   However they are probably installed within the Sonar path and perhaps there is a call/response to check their validity.  
 
Does anyone know if the "rental" adpaks run in the same shell application as the adpaks you own outright?  Is it possible to use the rental adpaks in another sequencer while your cakewalk subscription is valid?