I'm glad that I'm starting to get feedback like this. I believe you that more companies are doing work sample tests.
For the the gigs that asked you to do these problems: did you also get interviewed with a standard programming interview? If you had to guess, between 1% and 100%, how much of the weight of your technical evaluation do you feel was on the sample challenges you did at home, versus Q&A and whiteboard coding on-site (or on the phone)?
Don't remember details about interviews, but I'd tell that homework was about 75%, because in those two cases they told me that I exceeded their expectations and they learned something new from my projects.
My last job search wasn't typical (I thought of starting a consultancy, so didn't search for a job per se), but one before that showed to me that it's exactly what people do. A company that was looking for a guy to write mobile SDKs asked me to build an Android app with embedded webview and animate html stuff with JS based on gyro movement. A slot game company asked me to build a minimalistic slot game from scratch (thanks to them, I have a clean minimalistic and complete game on github now). Another game company asked me to build a small endless shooter — and I ended up being the only candidate who moved texture offsets instead of actually moving game objects to infinity.
Each of those tasks took from 4 to 8 hours, and 2 of those got me an offer. And of course, I used the same method when I consulted others on hiring decisions or hired people myself. So, based on personal experience, companies do that, and it works.