Its a good idea. Make your short vids available for free and anything over say 20MB they must join.

Code:
{if $video_details.space > "20"}Login{else}show the small clip here{/if}
If your buying content split a couple of larger vids up into 30 second clips and make these the freebies

Also you can make a little mod so you can have a pay per view (based on the playlist table), say 50 cents per video or download. This will make you good money