I can't really understand why you would want to keep a preview file as these are generally made to demonstrate the quality of the video/audio of a file so people can try before they commit to downloading the full movie, unlike trailers there not there to give you an idea of what the movie is about so don't see the point myself.
You say that 'folder monitoring now ignores "Preview" or "Trailer" directories' I don't bother with trailers, previews etc but by that I can only hypothesize that folder monitoring will ignore folders called "Trailer" or "preview" if this is the case and you want to keep those files as a preview then why can't you just create such folder and put them in?
If you want to create a folder by another name and want folder monitoring to ignore it just create a file, name it 'mymovies.dna' and add to the directory.
Whilst It would not be a feature that would be beneficial to myself I'm all for making MyMovies a more user customisable experience and would have thought it would be an easy enough feature to add to allow a user to like you say add a list of names that Folder Monitoring would ignore, I also don't think this would be as Brian said adding a feature that is for the sole purpose of supporting illegal downloads, as users could have genuine reasons for wanting additional directories that ought to be ignored, for example those ripping ther own blu-rays to .mkv may not want to embed the additional audio and subtitle streams they may instead want to add extra directorys for audio and/or subtitles.
Also whilst I am not condoning illegal downloading, like it or not, I would imagine there is a very large number of MyMovie users who do and maybe MyMovies would not be where it is today without these users.
my advice is given to help, my advice might not always be the best solution but is given to the best of my knowledge, my advice is not always approved by MyMovies and often isn't, if my advice ever suggests changing in any way a title from what is contributed on the webservice please insure no invalid changes are contributed, please read the following guides
Disc Title Contribution Guide,
Contribution Introduction,
TV Series Contribution Guide