I think birds in winter can use any extra calories they can get!
Those commercial blocks just have so much millet, cracked corn, etc. in them. yuk. We do add other things... just as you said, nuts, sunflower seeds... last time we made a huge batch because we'd come across some old dried fruit in the freezer that still tasted fine but had turned an unappealing dark color. The birds didn't care what it looked like, or if it was a little extra chewy!
