I generally tip. But not if they are 20 minutes late getting to the airport. Or if they have 'tude.
Both of these are common.
If they are late but apologetic and work hard to make up for it, they'll get tipped.
Also, if the van is a total POS, no tip. Even if that's the hotel's van, they can help keep it clean.
The van in PWM for a while had a door that wouldn't really close properly. And it was filthy. And they'd come late. From a mile away. 20 minutes to go 1 mile? "He's on his way" and 20 minutes later still no van?
No tip under those circumstances.
At some level it's still got to be earned.