Limitations Of Import

Synfire is able to read Standard MIDI Files. Import however, is actually a misnomer. Synfire is neither a DAW nor a MIDI file editor. Figure recognition, the process of converting static MIDI data to dynamic phrases, is a lossy and ambiguous process. Moreover, MIDI tracks are full of repetition and duplication you need to sift through and eliminate manually, since this task can't possibly be automated without making the imported arrangement sound even more different from the original.

A more appropriate label would be "Creating New Phrases From MIDI Files", because that's exactly what it does. Standard MIDI Files do not include the necessary meta-data that would be needed to properly import an arrangement and convert it to phrases that exactly render their original MIDI notes. This is an impossibility, as is explained here.

That being said, harvesting phrases is still one of the best ways to build large libraries quickly. Apart from generating phrases randomly with KIM Factories, that is.