I also agree with @Shampra that collection-specific relation serves different levels of functions from tags.
Collection-specific and Object Context are identicators for situational use of relations. For example, with a collection for the shopping list of a date, I wouldn’t want to create buy apple task for each purchase to track spending/prices across the whole year (that would mean 52 objects per year for apple alone if buying once per week) 
Cause each ‘buying task’ could also be reused/repurposed for bookkeeping / expenditure tracking.
And I don’t imagine anyone would use tag with date 
#Christmas is fine though…
And as a non-tag users 
, I would prefer to create objects and MOC, and then use object type relation for themes, categories and concepts… There are lots of contents and descrptions within each topic and they could continue to grow, for example ethics and metaphysics with Kant, London eye and boroughs for London.