Blanket sleepers are always great to have on hand. My son was born in July, and in August of that year in Lake Tahoe it turned unseasonably chilly. It had been so warm that he only had little t-shirts to sleep in. Thankfully we were able to find a store that had some blanket sleepers in stock at that time of year. Blanket sleepers with feet are wonderful for keeping little toes and torsos warm. Add a little hat and your baby will be cozy warm. Body heat is lost through the torso and the top of the head so it's important to keep those areas warm. I don't know about babies, but I know if my feet are cold I can't get warm. Socks tend to come off, so footed pajamas work better than socks for keeping little feet warm.
Blanket sleepers are often, but not always, made of fleece material which is designed to keep baby warm even if he or she kicks the covers off. A little t-shirt can be layered underneath for more warmth. My preference was always for one piece pajamas for my babies in the winter time. With two piece pajamas it seemed the t-shirt always worked itself up leaving the tummy exposed. Blanket sleepers were a favorite of mine for all these reasons.