To this point Brust has outplayed all his contemporaries in the system, including Irving.
Irving hasn't really had much of a chance to play. The restrictions on his contract and uncertainty of the lockout have really prevented the Heat from putting together a rotation to keep Irving in the mix. They likely wanted to wait and save those games until it looked like the lockout was coming to a conclusion, or knowing he was on the farm for the year, before playing him. That has been a terrible position for both the player and the team. The player doesn't get a chance to play, so never finds a groove. The team wants to win so doesn't want to turn to a cold goaltender. It's been just a bad situation for all those involved, except the other two goaltenders.
Your example includes two middle of the road players, while Brust has excelled at his position.
Really? The same Barry Brust that was in German league last year? Come on, get serious for a second. Brust has done nothing in his career except bounce around the minors. He's had a hot start, but anyone who watches him will quickly tell you that he's unorthodox and horribly weak on fundamentals, those crazy things demanded by coaches in the NHL. Brust at the NHL level would be a waste of time and games.
As for it being a risk, that would be his choice to take that risk wouldn't it. Its not like he cant sign an AHL contract with the Heat or any other AHL club after.
They can't sign another contract if they are hurt. That's the point. They would have to release themselves from a guaranteed contract to go to camp without a contract or insurance. They get hurt, they are done. No one touches them with a 10 foot pole because of their injury. This may be news to you, but you can't get a contract registered with any league without the player passing a physical. An injury is going to make passing that physical pretty tough.