The only reason I can think for requiring a player to take part in multiple games before hosting is to better understand balance of role distribution, as well as how games will play out. But overall, hosting and playing are so different that it's almost pointless to lump the two together. Which is more or less what Jumpluff said, I guess.
Eep and I just finished hosting our game. Overall, it was a success imo. There were maybe 2-3 big errors throughout the game (a couple of which ended up being mostly remedied thanks to one of the game mechanics), but never any problems with who was targeting who, or how the actions affected each other.
What Yeti said about new hosts not hosting alone is, imo, really important. In Luigi's Mansion, I think a solid cohost would have told Walrein he was an idiot for even considering letting Sam sub in, which would have eliminated the biggest fault of that game. And also might've made him reconsider having a Dickens that was that strong.
BT, I sorta agree, but there's always the argument that very little separates Beginner from Standard anymore, other than the types of players who sign up. My first game was a Standard and, while I think it was a bit ambitious for a first-time game, it worked. And the hosting errors throughout the game would've been pretty much the same had it been a Beginner instead of Standard.
As far as blacklisting shitty hosts... I would hope that someone realizes they're a shitty host and just stops making games long before anyone has to actually blacklist them. If you've hosted 3+ games that all failed massively for one reason or another than you should probably just reconsider hosting overall. And if you know someone's a bad host, then just don't sign up for their game.
For what DW said, I think something important is to not "split up" the updates between hosts. Eep and I were both pretty involved in every update, besides the couple where one of us couldn't be around. However, I knew that both of us are relatively detail-oriented, so it was never a big concern. Making sure both hosts are involved in going over the results/writing the update is pretty important.
For something Yeti was saying about how the results got messed up in MUDS, something Eep and I sorta unintentionally started doing in the sheet was just having "<verb> <user>" in the sheet of Night actions. It wasn't always something really consistent, but it was specific enough to differentiate between other roles nearby in the sheet. I dunno if that would've fixed the problem(s) encountered in MUDS, but it's at least one way to possibly notice that you're recording in the wrong spot.
And another thing. Players should also take it upon themselves to make sure their information is handled properly. If you submit to the hosts on IRC, make SURE that they respond and say that the info is recorded. If you submit to one host on IRC and the other is afk/offline, then send a PM to one/both hosts anyway. If you're playing in a game where the hosts are new or have made errors already, double-check with them before update to make sure that your action is what you think it is.
Really, I think some people are just more cut out for hosting than others. After playing in a few games and hosting one, it has pretty much solidified the fact that I much prefer game design/hosting over playing. Other people probably enjoy the gameplay more than taking the time to create a really good game. Hosting shouldn't be some great goal you're working toward through playing. If you're someone who can be thorough and you have an idea you think would make a good game (theme, experimental concept, or otherwise), then you should go for it.
Another thing to think about is who you pick as a cohost. I learned through Viking mafia that Eep and I worked well together. I have a lot of really crazy ideas, while Eep does a good job at pulling me back and thinking about a lot of things that I missed. So make sure you're picking a cohost who is a good balance to yourself before the two of you start working. If that means you don't go through the Request-a-cohost thread, then fine. PM a few people that you think would work well with you. If you don't really know anyone well enough then, well, maybe you should play a game or two and get to know the regular mafia players better, etc.
It occurs that I have no clue what I just said. Sorry, I have a monster sinus headache. Hope it made sense :x
Edit:
Snype and BT both posted while I was typing, and I can't resist temptation.
What KoK/Yeti are doing is a good option for Beginners, but maybe switch it around. Have the "new" host take the lead in design, while the experienced host keeps them in check. Sorta like what the Circ mods do, but throughout the whole process, since the mods can't be expected to find every fault in the game design when they're only checking it over at the end of the process.
And the "games by new hosts" category wouldn't work. That practically screams "game where mistakes will happen" and people would hesitate to sign up for it. :/