This is a big one. Our FM solutions are not built around the concept of recids so putting this requirement on the XML API is curious. Assuming we have typical relational data model where tables are related using primary and foreign keys then it would be much more convenient to be able to make delete and edit queries using a record's primary key. Without this ability we have to make two queries for each action – one to find and fetch the records recid and a second to update that record.
The option of using portals to manage related data is too cumbersome in most cases. You don't always want to pull all the related data down in a request and are often times only concerned with pulling discrete sets of related data at a time – creating unique layouts to handle each combination of parent record and related data sets through portals would be an issue to maintain.