As a goalie, i want my defenseman staying on his feet for this play. a one knee down that you can recover from, as GapToothed mentioned, is perfectly fine. But i dont want to see my defenseman sliding past me, helpless. Its too easy for todays players ( even in lower beer leagues, etc) to power stop, and now youre left on an island. On top of that, if it goes across the crease and i can make the save, theres virtually no chance im controlling the rebound unless the shooter puts it directly into my glove. To boot, im also wildly sprawling across the net and wont recover nearly quick enough to make any follow up saves. At least if my defenseman stays on his feet, he can clear or cover the net until i regain form.
Sliding was ok before it became commonplace for anyone and everyone to be able to bat the puck out of mid air. You see this more and more these days, so just taking away the ice itself doesnt warrant the commitment to being out of the play.