Obviously I'm a fan of filmmaking as sport - which I see as a bit different than filmmaking as competition, at least as SonnyBoo describes it, where you are making judgement calls on whether one film is better than the other.
To me the closest thing I can compare it to is running. A lot of people run, but not everyone who runs races. Even when you look at most runners that race they aren't trying to win the race - the race just provides a goal to work towards, and a measure of their progress, and a way to test themselves. They're trying to run faster than they did last time over the same distance, or farther than they did before, and running in the context of the race drives them to push themselves harder than they would if they were just going out for a run on their own.
In my opinion these kind of competitions encourage rushed, sloppy work. Sure, there are occasionally decent shorts produced during these competitions, but by and large it's a plethora of sloppy crap.
My personal experience with these (and I've got more than most - ~30 films of my own, plus I've watched probably 4-500 other people's films at screenings) is that they mirror the general quality spread of short films I see made outside of competitions. 80-90% of them aren't particularly good and could benefit from having a lot more work put into them. The other 10-20% are decent, and only a few of them are truly amazing. That's the same spread I see watching films in the IT screening room, or on youtube, or even in a festival context where I know I'm seeing the top 1-2% of submissions and sometimes find myself wondering how bad the rest must have been that some of the selections got through.
If you had enough time, and things went wrong, then you can mainly blame planning and try to figure out what went wrong. But in a timed competition, even Spielberg is going to have things go wrong. And you can always chalk the mistakes up to lack of resources and time. But if one can't blame anything other than him/herself, I think that filmmaker will get better quicker.
Things always go wrong, for Spielberg or anyone else, time limit or not - one of the key skills of being a successful filmmaker is learning how to adapt on the fly and make things work even when everything's going wrong. Timed competitions provide a unique training ground for that because you can't just stop when things go wrong and try again next weekend or whenever you can get around to it.
Decision making is another key skill - and when you're on a hard deadline it teaches you to make decisions quickly and keep moving forward without second-guessing yourself. The more you do this, the better you get at it, and the more confidence you'll have in the decisions you make in the future.
A hard time limit also forces you to learn effective time management skills - a key aspect of low budget filmmaking where you're trying to make the most of the time you have with the limited resources (people, locations, etc) you've got.
Learning how to manage a team effectively, get the most out of them in a limited time, juggle multiple responsibilities in the middle of the whirlwind of production, make the most of limited resources, scrounge up more resources on the fly, and on and on and on - these are all things you can learn, and practice, and improve upon by doing these kinds of competitions.
And the competition part gives you an easy context to practice these things - it's a limited time commitment for you and your crew (usually just a weekend) so it's pretty easy for everyone to make room for it in their schedules. If everything just really crashes and burns and you don't manage to finish the project you still get the experience but no one loses out too much. But if you do finish you've got one more film under your belt that you didn't have last week, and you can take everything you learned while making it forward into your other projects.
Plus - at least with the local 48's and similar competitions you generally get to see your finished film on the big screen in a packed theater within a week or so of finishing it. Watching your own work with an audience like that is one of the best learning experiences I can think of - it closes the feedback loop in a way that nothing else can. You get immediate, involuntary and visceral feedback on what worked and what didn't, while the decisions you made that led to the finished film are still fresh in your mind.
As for excuses - everyone's always got excuses for why their film isn't perfect. Can you use time as one excuse? Sure - but then when you're in the competition and at the screening, your film is up on the big screen with 10-15 other films and some of them are better than yours, what good is that excuse? Everyone there had the same amount of time, but if you weren't able to do as well as everyone else then time clearly wasn't the problem and you have only yourself and your skills and planning to blame. So you just have look at where you fell short and try to figure out what you could do better next time - whether that next time is another competition, or just your next film project.
So yeah - approach these things as merely a race where you're just out to win and it's probably a waste of time. Take them as a training ground, a way to test yourself and your capabilities against a set of limitations, and they can be of real benefit. I'm not saying everyone should do them, but I think almost anyone could benefit from the experience if they approach it in the right way.
I also never understood the 48hr/24hr film festival. Yeah, let's go to picasso, give him a limited number of paint, a torn canvas, and time him. ridiculous.
Would Picasso turn out crap given limited time, paint, and a torn canvas? It might not be as elaborate as some of his other work, but it would still be a Picasso. How many camera threads do we have around here where someone drops the "Spielberg's camera" line? Having more time won't necessarily make your film any better than having a better camera will - it's only your skill and effort that let you take advantage of either. There's always limitations in filmmaking, whatever level you're at, and there's someone out there making it happen within the same limitations. Doing the best possible work within the limitations you can't escape is what indie filmmaking is all about. The truth is, if you can't make a decent short film in 48 hours it's not because of the time limit.