If Yao comes back before the end of the season, it doesn't affect anything; there's no penalty for having Ariza on the roster.
That's the key thing. I don't get the logic, but the league has essentially given Houston a second MLE to play with.
It's not something that is automatically granted and there are cases when it hasn't been. Examples include Cuttino Mobley for the Knicks this year and Grant Hill for the Magic in 2003-2004 (when he ended up missing the season). A league-appointed doctor independently reviews the case so it's not like Houston's medical staff can lie in order to get another player and it sounds like the NBA generally uses a high threshold. I think this should be taken as a sign that there is no way that Yao Ming recovers for next season. no matter what the Rockets might say, and any exploration of alternative treatments is just desperate optimism.
Also, to answer what someone asked earlier in the thread, the disabled player exception is half the disabled player's salary or the average salary (MLE), whichever is
less.
If I were the Rockets, I might consider saving the MLE for when the mid-season buy-out-mania occurs. Spending it now puts the Rockets into the luxury tax due to Ariza's contract (the disabled player exception grants no relief on that front). So if they don't get who they want, instead of settling, maybe they should just see how the roster shakes out and put themselves in position to offer the most money for a player who gets bought out if they think there's enough to make a serious playoff run.