I certainly wouldn't want to underrate the training aspect. I approach TV scripts with much more confidence because of the experience of writing half hour comedies for the radio. It's meant that I'm not as being freaked out by having to think of something new six times over, or cover 35-odd pages with coherent dialogue and at least 100 jokes. Most script problems I've encountered before - writing Hut 33, or Think the Unthinkable. I'm used to hitting deadlines, taking notes and producing new drafts.
I've also met other writers through writing for radio comedies. This was especially helpful early in my career when I needed reassuring that nobody really knows what they're doing. When I was starting out, Weekending was the show new writers gathered around. Now it's Newsjack. Meeting other writers isn't essential but it can stave off insanity. Writer-performers get to congregate in Edinburgh for a month every year. But if you're not a writer-performer, you don't necessarily have a place to gather and TV doesn't really have an equivalent. Plus there is the chance that you might hear your joke or sketch going out on national radio. Or in front of the audience. That can be a real shot in the arm.
But radio is an end in itself. It’s a great place to write. The medium itself is certainly intimate. It’s more like reading a novel, where the pictures are in your head – where the special effects are so much better, and far more memorable.There is more to be said. And I have done last February: here. And I mention it because I'm running a day on writing comedy from the radio in London with Dave Cohen on Friday 23rd November 2012 with another on 30th November about writing sitcoms for TV. More info here. Sadly it’s not free or funded by the licence fee, but it's reasonably priced. If you’re looking for some tips on where to start, or how to progress in radio, this is definitely worth a look.