Nitpicky, but ActiveRecord::Relation is not ARel. ARel is the relational algebra library that underpins ActiveRecord (>=3.0). See http://erniemiller.org/2010/05/11/activerecord-relation-vs-a... for more info.
The specific example he used to demonstrate worked transparently because of the has_many :through relationship for users on Article, which requires the collaborations table to filter the users, so joins it. Otherwise, you would need to do the join yourself before merging the scope, and things get messy pretty quickly, especially if you end up with table aliases (which the merged relation knows nothing about -- it will still query against the collaborations table).
I added "sifters" to Squeel (http://github.com/ernie/squeel) in order to address the need for a set of reusable conditions on a specific model that could work through an association. I'm not saying Squeel is the solution you're definitely looking for, but I just want to let people know about the things they'll need to look out for when using Relation#merge.