I'm just not sure doing spec work on one of those sites is a good way of "bettering your talents". If practicing and honing your skills is what it's all about, I can think of a bunch of ways that are better than participating on those sites. I might create a fictitious brand, something that is really challenging or something I have never done before. Let's say I want to do more beer label designs, but beer companies aren't knocking down my door, create a fake brand, practice on that. This way you aren't restricted by a client with bad taste and a dirt cheap budget. The sky is the limit, you can really challenge yourself and then you will have something amazing for you portfolio that shows people you can do that type of packaging design. This is how you develop a niche, or target the type of work you would like to have. If you have a portfolio full of nothing but 15 minute $50 designs, guess what? That's the type of work you are going to attract when people see that portfolio.
Practicing or not practicing, why just give away your ideas to a company that should value their image and branding enough to hire professionals so that those professionals can spend the needed amount of time to create something unique and that works? If a person wanted to donate their time and expertise, they should create some designs for a charity they believe in, or how about something that they can swap with other fellow designers. Anything but spec work.
Spending 15 minutes scraping something together to compete against people who are also spending a few minutes drive-by-designing, or worst, who are just out their ripping off other designers, isn't challenging IMO. You wouldn't be "bettering your talents", unless your talents are throwing something together as fast as you can. Those particular talents won't necessarily serve you well when you are working with a real client who expect well thought out and well executed final results. If those other designers were really good and challenging to compete against, they wouldn't be on those sites making pennies on the dollar, they would be to busy selling real jobs and making real money.
While some folks are wasting time "in the trenches", "observing" and at best winning 5-10% of the contests they compete in and giving away 90-95% of their time, they aren't leaning the other major part of being a designer, and that is the art of selling it. In fact, by competing on those sites, their actions are actually counterproductive to "bettering your talents" as a seller of design.
So this isn't about whether or not I can except that spec work is and will continue to be a reality. I except it, just like I except that there will most likely always be mosquitoes in my lifetime... but that doesn't mean I going to go and try to get bit by as many of them as I can, and I'm certainly not going compete on one of those sites. If we as professionals can convince just one designer that they are wasting their time on one of those sites then that's a good thing.
