Long story short, the argument for 99designs is that people who wouldn't want to pay for real designers get what they want/deserve. The argument against it is that it belittles the industry and the designers themselves because people come to expect that that's all they should have to pay for this type of work. Additionally, having so many people clamoring at the chance to work on such cheap projects makes it seem as though this is a popular/acceptable/financially secure way to make money in this industry when it isn't. To go into detail..

Some people are always going to be at a skill level where 99designs is really the only chance they have to get paid for their work. These are people that despite all of their years, just don't have the eye or the execution to improve. At the same time, they're going to feel as if over these years, they'll have accumulated enough work and had enough experience to charge more for the same, crappy stuff, so you'll see people charging higher rates for subpar work simply based on the fact that they were able to get away with getting paid for crap to begin with. These same people, despite their work, will have attained high reputations and have more projects under their belt, which makes them look better on-paper than someone who is actually a better designer. Worse, there's a lot of folks like these who take someone else's initial submitted concept and run with that instead of iterating on an original idea. This gives them more time to work on the details of someone else's submission, thus looking more fleshed-out and more likely to be chosen.

For others, it is a jumping-off point, where they contribute to help get a feel for real-life projects, client interaction, and just to hone their skills. These are often teenagers, college students or people making a career switch. The difference between these and the above is that they do get better, but they might feel as if they're going to upset past clients if they begin charging what they're worth.

Additionally, there's no 99designs for a higher caliber of designers. There used to be a service called Arteis (a private subsidiary of Logoworks) that worked in a similar way to 99designs (crowdsource-to-one-on-one), but they would kick designers who they felt weren't quality enough, ignored the site or were rated poorly by multiple clients. A couple years ago, they closed the Arteis service down and made it so that you were either working internally for Logoworks or you were one of their on-call freelancers. I had friends that were making upwards of $30-50k a year off of this service who went from having a seemingly endless array of projects they could work on at their feet to having to hustle to get work at all (especially at the rates they were charging through Arteis, because being vetted by a company like that made clients feel more confident in the money they were spending).

This is the same problem 99designers often find themselves in, too.

And to your point about being happy with the results, I think that's subjective; if you're not a designer yourself, it's difficult to foresee the application of the logo working everywhere it will need to. Most 99designs logos are noticeably so, and honestly when I come across them in the wild, the company/project probably would have been better-served with just plaintext until they could afford a professional.