I definitely agree. I don't know how drives are conducted where you're at, but here, after a person has donated, there's usually a table filled with all you can eat food and drink. Sometimes it's Chinese food, sometimes it's snacks (Rice Krispie Treats are my personal favorite). And they encourage you to stay a while before taking off.

Still, it's important to take it easy for the rest of the day. Even after stuffing my face with food post-donation, I've had a number of dizzy spells. Everybody's bodies replenishes the supply at different speeds.