As I've previously stated, I do tip around 15% because the driver is dependent on it, but man, the tipping system is such a fucking BS system.
1) It puts the onus on the customer to, essentially, punish the employee. Because the customer is also at risk of having their food messed with, this is inherently a very flawed system. Like was previously stated, do you think an employee is going to understand that they did poorly and take that into account? Fuck no; they'll hold a grudge and, if they're of the type, mess with the food/deliver last next time.
2) It also puts the onus of punishment onto someone who won't know the whole situation--if the driver was made late by the place being busy, a screwed-up order in the kitchen, flat tire, etc.--All things an employer would have the time and ability to understand over the course of the employment. The customer wouldn't, and may not tip well because of it. Here, the employee is being screwed over, as well.
3) Cheap or "principled" customer will not tips, regardless of service quality. This, once again, screws the driver.
The only person it really helps consistently is the business: they get to pay drivers a shit wage and pass the delivery charge directly onto the customer. BTW, fuck you if you charge $5 for delivery and pay your driver $0.25 of it or $2/hour. You didn't drive it there, asshole, so don't take the pay for it; if you expect me to pay for your driver directly, don't make me do it twice. Because of this, the business gets to maintain the appearance of "affordable" pizza and still not have to pay their drivers a decent wage themselves. They couldn't give two fucks if the employee gets tips or not, which means they have no reason to make sure they're paid a fair wage. It's BS.
There should just always be either a flat rate or increased prices for delivered items. That way, the customer is not put in danger of being treated poorly, the employee is protected from being wrongfully screwed out of a tip, the drivers would have base wage to live off of, the employers would be forced to pay a decent wage, and the cheapskates will have to pay their fair share. With this system--assuming it's a flat rate that goes mostly to the driver--a good driver would still make more money than a poor one. It's win-win. And if a bad driver is consistently under-performing, the onus is on the employer to dole out "punishment" as required--someone who is not nearly as vulnerable as a customer getting food delivered.
Just my 2c, though.