In my time as a game designer, I've lead a few sub-teams and design teams (8 total). Each team has included at least one brand new, junior designer with no experience. I think this is kind of important - I think new designers bring something to the mix of designers in a team that you can't get any way else: Passion unmitigated and not yet beat down by experience.
We love to talk about how much experience matters in this industry, and it does, but what we fail to mention when we talk like that is that all the years of working in this industry, they wear you down. You start to censor your own ideas before you ever air them out for people to hear, because you think to yourself, "That didn't work on project X" or "No way engineering can pull that off" or "There's no way we'll get the art for that."
When you're the new guy, those concerns don't exist. It's a huge open world of "All it takes is a little effort and we can do anything!" and that can lead to the type of breakthroughs in your game that can be downright brilliant...
There were several new designers on Auto Assault, for example, and each of them brought something new and interesting to the game. From clever uses of simple things like Quest Pre-reqs to build branching storylines or unique ways of looking at the scripting tools to create complex AI all the way to fundamental things like approaching level and mission design from a different viewpoint and thus bringing us missions we couldn't have had, any other way.
On Nightcaster, an old Xbox game I worked on, it was the same way -- our new designer used the mobs in ways I had never intended... ways that made the game a LOT better than it could have been.
Don't get me wrong, it's not like anything they did could never have been done by a designer with experience, but an experienced designer can get set in his or her ways... a new designer has no "ways."
Another plus to the new guys is that you don't have to "break" them to get them to work, your way. I do make the assumption that the FNG you hire isn't some overbearing, super-opinionated freak or uber-fanboy, but generally speaking, new designers want to learn. If you ask them to write you a quest and then show them how it's done, they'll take what you've said to heart... They won't just return to what they did last project, the second you turn your back...
...Of course there are huge risks too. The new folks have never done the "deadline looming" thing. They may not have the psyche for the job. As much as we want to avoid it, crunch can happen and the new guy is a huge, anxiety inducing question mark, when it does. A good game designer has to be able to work independently and react quickly to changes in the game necessitated by the results of iteration, too -- something that may be difficult for someone still overwhelmed by the position itself.
And, it's not like I'm saying experience has no purpose either. Far from it -- there is HUGE value in experience. Even returning to what you did last project, when you're left to your own, is likely done because you have the experience to know that what you did last time will work and work well. Experience teaches you tricks and hones that "gamer's gut instinct" that lets you know what's going to suck and what won't. My point isn't to say new "guy > experience," because that's crazy talk.
No, what works best is a mix of the two. You want strong, experienced leaders with many projects under their belt at the helm, and below them you want experienced seniors... but at the "grunt in the field" level? Maybe experience isn't what's most important.
Maybe, what's most important is the passion to do the job, the genius that comes from being allowed to unleash your creativity for the first time and the brashness necessary to use that genius regardless of the risks. Experience teaches you the risks in all of the above, and that's good, but maybe if we let it happen more often - and temper it with the experience in our design leads and senior designers, we'd have more interesting games, as a result.
Me? I already know the answer; I'll keep hiring the new guy on every team I'm able. I haven't been let down, yet.