This is planned for better support of MediaViewer, but there doesn't seem to be any ticket about it yet.
Thumbnail generation is slow; for MediaViewer, thumbnails which have not been rendered yet take almost twice as long to load for the average user (an extra 700ms). If the original file is very large, thumbnail generation can take ridiculously long (although bug 67525 will address that). To provide good user experience, we need pre-generated thumbnails in some standard sizes, which frontend applications can rely on.
A related, but outdated, RfC: