'just' a movie player?
Quicktime Pro lets you edit, basically, ANY type of quicktime media. It is EXTREMELY powerful.
Now it's ALSO an mpeg4 authoring tool...
With quicktime pro... you can compile still frame animations, edit clips of movies together, composite video tracks, add titling, graphics, splash frames, subtitles, compress videos for the web, explode video into image sequences for editing/wire removal, and many many more things.
Basically... you can squeeze much more functionality out of quicktime pro than you can out of iMovie.
Besides... the player is free. If you don't want all the extra stuff, you don't have to pay for it.