The nice thing about frozen veggies is that they can save a trip to the store if I keep several kinds on hand. So if a recipe calls for a cup of corn or a package of spinach, I often don't need to run to the store if I have it in the freezer. I had so many extra peppers last summer, I sliced them and froze them and we've enjoyed them in fajitas all winter long.
I use quite a bit of frozen corn as we can't get good fresh unless in season (mouth watering for fresh from farm sweet corn!!!). I keep a large bag on hand in the freezer for a quick side dish. I also buy frozen green beans to have on hand as in the winter, the fresh are not in good shape and are expensive. When mixed veggies are on super sale, I buy a bag or two for use in a quick veggie soup. I also keep a box or two of frozen spinach on hand.
I prefer fresh and try and buy seasonally so I get the best bargain for my shopping dollars.