The "Filter" property of TDataSet was I believe introduced in Delphi 2 as a TDataset property (at least I can find it documented in my Delphi 2 reference guide - but not Delphi 1). It made sense when you were dealing with Paradox (or similar) tables on a local disk and a simple syntax was defined for it looking like a single level SQL conditional statement. I assume that the filter was interpreted rather than compiled.
When you are working with a true SQL Server then it is difficult to see why you would want to define a client side filter. The SQL Server is always going to be able to perform more complex and much faster filtering of datasets; you also avoid the communications overhead of having to send records from the server to the client, only for them to be discarded by the client side filter on arrival.
Not every possible filter can be defined in SQL - and that is why the OnFilterRecord event handler exists. In such cases, client side filtering is justified, and the code is written in Pascal and compiled. This is very different from interpreting a simple condition clause in a filter statement.
IBX is solely concerned with delivering an optimal driver for a Firebird (SQL) Database. It has an internal SQL parser that allows for modification of the SQL select statement every time a dataset is opened. In the BeforeOpen event handler of any IBX dataset, the the user can readily add conditions to the WHERE or HAVING clauses, or revised the ORDER BY clause to change the sort order - as well as more radical manipulations. This feature allows for efficient and advanced server side filtering in response to (e.g.) a user selecting a check box on a form, or clicking on the heading line of a DB Grid to resort the table. Indeed, this can all happen transparently when using the IBX versions of the DB Combo Box, DB Grid and Tree View.
It is perfectly conceivable for the IBX SQL parser to be used to extract a simple condition from a filter property and then interpret this. However, no user has ever requested this and I can't see why it would be worth the effort to add what is always going to be a legacy feature.