One person's "waste of time" is another person's "building atmosphere and paranoid tension". Sure I wouldn't like it if this happened lots. But happening once in a campaign (and not every campaign) is fine.
It's the sort of thing that players/PCs will refer "fondly" back to later, "You want to sneak into and ambush the bandits in their lair? You remember that time we did that in the Waterdeep and the bastards weren't even fecking there. Are you absolutely sure they are here..." It becomes part of the lore of the campaign/playgroup. You don't get that effect from one sentence "You search the tower and it's empty".
Oh, for sure. I wasn't meaning to imply that he was doing it wrong or it was wrongbadfun. Not at all. Just differences in tables. I would absolutely lose my cool if a DM did this to me. But, then again, we play very short sessions (about 3 hours per week) and it's not unusual for use to have breaks in there as well. So, "wasting" (in my mind, this is wasting, thus the scare quotes, I totally accept that other people view it differently) a large chunk of a session only to have it be a non-issue is not something I would enjoy. I totally get that others might think this is great. For me? I'd lose my freaking mind in frustration.

[MENTION=996]Tony Vargas[/MENTION] - for me, and again, I'm only talking about myself, if the DM feels the need to hide things in order to run an entertaining game, I simply do not want to play at that table. I do not like it. Not that it's wrong or bad or anything like that. It's that it's wrong
for me. I don't play that way. Heck, I'm not even a big fan of Fantasy Ground's dice tower for hiding rolls. I grit my teeth and accept it, but, it bugs me every time the DM uses it. I'm not a child. If I roll a poor Diplomacy check, I'm perfectly capable of role playing that. If I miss and the baddie crits me and kills me? Fantastic. I'm a happy camper.
I used to have a sig quote from a long time ago that went something like this: "In D&D, the DM provides the script, the dice provide the direction". To me, that's exactly right. The DM sets the event, sets the mood, does all the things that you would find in a script for a play, and then the dice are the director, telling me what I should be reacting to. I have zero interest in having the DM be both the script writer and the director. And that's true for me whether I'm playing or DMing. I implicitly trust and expect my players to play out bad rolls and deal with bad luck. That's part of playing the game. In exactly the same way that I implicitly trust them to play out great luck as well. And it works for me.
For me, (and I cannot stress this enough that I'm ONLY talking about myself) fudging like this shows a lack of trust on the DM's part that the players cannot handle certain situations in the game. If the DM truly trusts the players, then there's no need for fudging whatsoever. At least, that's the position I'm working from.