Originally posted by: iRantanplan
Well, AVISynth is a frameserver, so in theory, you can use an AVS file with every program that can read AVI.

If only it were so... but there are many programs that don't support AVS files. I guess AviSynth won't work with programs that read the files literally (I don't know the technical details of AVI files or the code used to parse them, but it would be great if someone else could follow up on this). That's why we need to use a plugin in order to use AVS files in Premiere.