This admittedly is an impression based on incomplete information, as I didn't put my hands on the full version. Minispillage is a plug-in instrument audio unit or VSTi, that uses synthesized percussion and attacks. For anyone who does not want to enter the "sample race", buying and collecting libraries willy-nilly, synthesis offers you another strategy, keeping firm grip on the qualitative decisions you make as a musician/orchestrator.
This mini version leans more on the retro sound to begin with, and most likely will not replace your favorite kit. That's why it's a bit difficult for me to evaluate its final price tag should you wish to get the full version. It is meant to give you a sound check to the synthesis engine, and to discern whether you really want the full kit.
Another reason to go synth over sample is improved responsiveness and decreased CPU and RAM overhead of your sessions. If I recall, the native plugins, simpler and ableton's drum rack both placed less CPU burden, and subsequently less latency on my sessions than did mini-spillage. I'd really like to see mini-spillage re-released as a drum kick replacer, more robust in parameters, more stable, while dropping synthesis for wood block, snare etc. Then people could really put it through the ringer.