The use of an ECHL affiliate is all dependent on how the NHL parent uses their AHL affiliate. Take a team like the Manchester Monarchs, the Kings use it primarily to develop talent. If the team does well, great, but the focus is not on winning the Calder Cup. Thus, there are usually one or two players who are not Kings property on AHL only deals. There are usually a couple Kings property on AHL deals with the understanding that they are on them because there is no room under the 50 contact cap.
Now you compare that with a team like the Hershey Bears. The culture of the Bears was to build Calder Cup Champions. Thus, the Washington Capitals, have allowed them roster spots to do so. Basing on the roster on their Wikipedia page (easier than trying to figure out who is on a Caps and who is on a Bears contract), they 8 players on Hershey only deals.
So what does this do to their ECHL affiliate? It means that the Ontario Reign are basically an affiliate of the Kings in name only and generally carry one Kings goalie, who is generally a project. This year they sent Maxim Kitsyn down too, which is unusual for a skater to be sent to Ontario. This leads to a more stable roster in Ontario, where typically the only roster moves are to improve the Reign.
On the flipside, in Washington's case, it means that their ECHL team is stocked with plenty of Capitals or Hershey Bears contracts. Meaning that their ECHL team will likely see more fluctuation based on the prospects shuffling between DC and Hershey.
So yes a 30-30-30 system may be the goal. But realistically, the ECHL is barely utilized by some NHL teams. So yes it helps the ECHL club with promotions to say the official ECHL affiliate of NHL Team X, but beyond perhaps a goaltending project and the odd visit from the parent's goalie coach, there is little involvement with the ECHL.
Compare that to the baseball system, where I don't believe independent contracts are allowed at all. From the MLB club right down to the Dominican/Venezuelan Summer League Rookie ball teams, every player is under contract with the MLB club. Even if there's a player available that could contribute to say the Rancho Cucamonga Quakes' California League run to the pennant, the Quakes cannot go and sign him, unless their MLB parent team does. Of course, I don't follow MiLB that closely, perhaps Rancho Cucamonga would say we'd like Player X and the Dodgers will acquire said player for them on a one year deal and release one of the players that stink, I'm not totally sure. But at least from my cursory following of the Washington Nationals system, it certainly doesn't appear the MiLB teams have any say in what players get assigned to them. It is 100% developmental, which is how the LA Kings treat the AHL.
In the LA Kings case, on the note about the parent club picking coaches etc. the GM of the Monarchs, has typically been the Assistant GM of the LA Kings. Thus, he does have the ability to hire and fire the AHL coaching staff and I believe when Ron Hextall held the position he did. Of course, being the Kings' Assistant GM means that any Monarchs AHL only deals he signed were likely on the advice of Kings' GM Dean Lombardi. You want your prospects to develop properly, you bring in the right teachers.
You actually find this mentality to an extent with draft choices and the Canadian Hockey League too. I know more than once in the WHL (I don't follow the OHL and QMJHL to know if it is similar), Prospect X's parent team forced him to be traded from a bad WHL team to a good WHL team. I believe just recently that was the case with the Oilers' Leon Draitsil (sp?). I believe they were not intending to send him back to the PA Raiders and forced the Raiders to trade him.
It is also sometimes why superstar Europeans end up in the Canadian Hockey League. I know when Anze Kopitar was drafted, he upset the LA Kings by deciding to stay in Sweden, instead of coming over and playing for the Regina Pats in the WHL, who the Kings promised he would show up. Thus, the Pats selected him in the CHL Import Draft and he never showed up.