I think it would have to be left to the conscience of the individual player. When I joined this site I had previous Diplomacy experience of about half a game (a disastrous attempt by five of us at uni), so I was about as noobish as you can get. Other people new to the site can be seasoned players of many years despite their blank records. Personally I stopped defining me as a noob after I got a couple of wins. I've now promoted myself to the dizzying heights of "enthusiastic bungler".

But in terms of replacement slots it's better to have anyone than noone at all

As long as the country in CD isn't in a dominant position, it's fine. Arguably you're helping, by coming into a situation that no noob would be willing or able to take.

If you would like newer players to fill up slots (either to fill up slots or to enrich the noobs) then offer a small points incentive which are important to new players but less important to the hardcore players.
The amount of games that a new player can join rightfully restrict him to playing only a few games. Perhaps make them able to join a certain number of games with CD countries (a base limit of three), or make it so that the game that requires a smaller amount of points to play, or maybe offer a down payment of points.

If you would like newer players to fill up slots (either to fill up slots or to enrich the noobs) then offer a small points incentive which are important to new players but less important to the hardcore players.
The amount of games that a new player can join rightfully restrict him to playing only a few games. Perhaps make them able to join a certain number of games with CD countries (a base limit of three), or make it so that the game that requires a smaller amount of points to play, or maybe offer a down payment of points.

Personally, as a noob, I usually use that kind of situation to my advantage by demonifying the experienced newcomer as a rallying point for those already in the game.

Depends on how expierienced they are: I was in a game where that happened with a large country, and the replacement had 1000+ points. He kinda threw things out of whack, which I found mildly irritating (it was my first time playing France, and I was winning!).
But it IS good to help out n00bs with strategy; I suggest starting a "School of war" game or something. One expierienced player and six inexpierienced players.

So ya, hold the slot for n00bies.

i dont think it matters. the ratings on this site are vague at best. someone could have a high GR and 9k diplomacy points and then enter a game costing 8900 diplomacy points and win. their GR stays similar but their ranking on this site drops heavily.

also, someone with high points on this site could have just survived every game with 5 or 6 centers and still gain points every game if its PPSC.

lastly, many players welcome the new addition to the game regardless of skill level, especially if the CDing player was the ally to another guy who is now left with no allies and a growing enemy.

"high GR and 9k diplomacy points and then enter a game costing 8900 diplomacy points and win"

loose, do you mean?

lol, yes i mean 'lose' not win.

I think it's fine to enter a CD irrespective of the experience level of the other players, because the longer a county is in CD, the more annoying it gets.