On preSetData data send to the event is not processed by modelTransformer. The easiest case to test is creating collection type with attached model transformer.

Data bound to the event is original data instead of transformed one!

This is required for example when transforming collection from array('key' => 'value') format to array(0 => array('key' => 'original_key', 'value' => 'original_value')).

If collection receive 1st format (wrong - not transformed) instead of 2nd one (good - only model transformed) it created bad names for inputs (based on original key, instead of on transformed ones)