Most freelance writers will tend to focus their efforts on one medium: novels, computer games, film, television, radio, comics, journalism or another area. That's not to say many of them don't work in multiple areas. Depending on a single medium for your living is dangerous. If that medium is seasonal or prone to ebbs and flows, so will your income be. [The worst thing a freelance writer can do is depend upon a single employer in a single medium. If that goes south, you're sunk.]
Different media offer different opportunities. You can make a very good living writing computer games, but chances are you'll never get to create a new game that gets published. Write a novel and you have a near direct conduit to the imaginations of your readers, but not many authors survive on advances and royalties alone. Get a regular television drama writing gig and hefty five figure payments will follow. But not all TV writing experiences are happy ones.
I like the variety of tackling different storytelling media. Each has its own particular craft and skillset, so mixing and matching provides fresh challenges. You'll learn a trick while writing a novel that can influences your next radio play, for example. Writers have a fount of ideas bubbling up from their imagination, some ideas won't fit their current medium. No matter, file it for later usage.
One thing I notice from talking with other writers is those who focus their efforts in one area often find themselves pining for a change. Write comics long enough and you run the risk of getting burnt out. It's not that you don't have stories to tell, but the prospect of writing them in comics form fills you with dread. Mix it up, shift focus to another medium - novels, computer games, whatever. If you're bored with what you're writing, that will infect your audience.
Cross-pollination is good. Several US TV writers spent the recent strike crafting comics and graphic novels, flexing their storytelling muscles in a different medium. More than anything else, a writer writes. But just because you're a celebrated writer for one medium, doesn't mean you automatically have the skills necessary to excel in another. Every medium is different, taking time to grasp - even great storytellers can stumble outside their comfort zone.
Sometimes a particular medium can feel more like a ghetto. Comics writers often feel they don't get much respect, especially from writers in other fields. For a long time film writers got accused of sneering at TV writers. So-called literary novelists get invited to festivals while pulp fiction scribes are dismissed as hacks. Poets are seen as artists, yet get the worst pay of almost any writers. No matter what ghetto you work in, the next one always seems greener.