My main point against a single prime lens for film making, is that it cuts down your options. A standard prime lens kit for film usually consists of five or six lenses - and that for a reason. You need these to be able to tell your story - to jump into a close-up without having to move the camera(requires a long lens), to narrow down the background to frame into behind your subject (requiring a long lens), to force the right depth of field (long or short, depending which way), to do a travelling shot without a big heavy dolly or a steadicam (requires a wide angle, or it'll be far too shaky), to take an epic landscape opener (requires a wide angle). What's more, if you're shooting ad-hoc (stills or video) a prime lens is a pain, even if you are carrying a lens set around with you. If you see something interesting on the other side of the street, chances are that by the time you've opened your lens box, changed the lens and stood back up the opportunity is gone. So, if you're on a budget, the big advantage of a zoom is not the ability to zoom or necessarily the quality of the lens, but the fact that you only have one lens to lug around with you (and to buy of course).
My feeling is that at a certain budget level, there's not much about the set-up that the user can control. There's the choice between a bunch of DSLR h.264 cameras, that are all pretty similar, there's no budget or scope to put any production value in front of the lens (lighting, art direction, actors, subject matter etc.) or even to move the camera or put it anywhere unusual (jibs, cranes, dollys etc.) For this reason, a huge pressure comes onto the lens - as though the lens alone makes a great image. This isn't true. You can take great pictures with a crappy instamatic camera, if you've got the right eye and the right attitude.
My personal recommendation is this. Buy an adequate kit with an adequate all-purpose zoom. Spend the left over money on things like a decent shoulder rig, tripod - or even a light or two, that will further improve your versatility and ability to create different types of footage. Once you've been shooting a while, you may feel that you need a prime lens, and you'll almost certainly have a good idea of where. I suspect that initially it'll be at the very wide or very tele end.
Incidentally @Painterman and anyone else listening, in case this isn't already clear.
A 50mm lens on a full-frame camera (36x24mm chip size) will have a certain field of view. You can imagine that if you were to take a smaller chip and stick it over this chip that a smaller part of the image is going to fall on the smaller chip. That effectively means that a smaller chip produces a smaller field of view for the same lens. That's why when we're talking about cameras like the 600D or 7D that have a chip the size of a frame of 35mm movie film (where the image is perpendicular to the film. approx 22x16mm ) and a frame of 35mm stills film (where the image is parallel to the film and hence substantially bigger), a given lens will have the equivalent field of view of a longer lens on a full-frame camera.
This is something to watch out for. If you buy a 50mm lens for your 600D or 7D, thinking that it's a standard portrait lens, you'll find that it's rather more telephoto than you expect when attached to your 600D/7D.