I've never had the idea that Open Source Software was just a play thing. Look at Apache, Android, Firefox. Are those just toys?
I can think of a few reasons that people think Open Source is just a play thing though:
1. Open Source Software is a wide spectrum, which means it includes software ranging from very mature to hardly usable. People tend to just remember those "hardly usable", and forgetting about the very mature ones (because they don't think of them as Open Source).
2. Open Source means there are a lot of minor releases. With proprietary you never see those releases because they are not released to public.
3. Much Open Source software is developed by volunteers, which means development is slow (because it depends on the developers' spare time). This means features might take a long time to become included because they are boring and time consuming to develop. A paid developer works on both fun and boring features, but a non-paid developer won't work on the boring parts.
But number 3 is not a law of nature, there are paid developers working on Open Source Software. But not on all Open Source Software.