I think the point of reference here should be your behavior in a bar. I think it's inconsistent that people who will leave $10 in tips at a bar over the course of an evening (three drinks/ two people would be typical) won't leave a single penny when they come in to pick up a pizza. I don't think it's the world's biggest secret that those bar patrons who tip heavier expect to be served a more generous portion, so to speak, and they expect to be favored in a crowded bar when they want a refill. To me, tipping the pizza place when you pick up a carryout order is the same thing.
I view tipping as a barometer for the service you receive. When you pull up to a drive-thru, there is very little service involved because drive-thrus in fast food restaurants are designed to mimic assembly lines. There is a point of contact at either end of the transaction, and everything in between it's do-your-bit-and-move-it-down-the-line. So no tip.
When one places a carryout order, one is trusting that
1) the contact on the phone will get the order correct;
2) the order will be faithfully communicated to the cook;
3) the cook won't flub it;
4) the cook will finish the order in a timely manner;
5) the order will make it to the register shortly after you arrive and it will be packed reliably, hot, and delicious.
If this set of requirements is met, then the staff deserves positive feedback. In a restaurant, that means tip.
It's probably clear that I was once a restaurant worker myself. One other word to the wise: restaurant employees DO remember names (even phone numbers sometimes!). I've delivered food in small towns and in the big city. This fact does not vary. If you didn't tip last time, you're not gonna get that pizza in time to make it to feed the kids AND make it to the opera. Positive feedback is the name of the game.
boy wonder