When mapping data in side Designer, a user is forced to bring in all dependencies (think tables with foreign key dependencies) in order to have a valid model. You should be able to override this - make the user aware of the missing data and allow user to accept the limitation.
Think of a scenario where there is a large database, with say 100 tables. Tons of foreign keys, etc. If a user wants to create a model against one table that has one foreign key (and bringing in its dependency brings another foreign key....etc) - if the user doesn't need the information from the related table for their modeling, then they should be allowed to override the warning and model without the dependency.
To reproduce - create a relational store and create 10 tables (A - J) - A has a foreign key to B, B to C, and so on. In designer, be able to override the warning when trying to model against just A.