My post on this topic got lost...
These racks were as common as dandelions outside elementary schools in the late 50s through the 70s. Kids' bikes had 20" or 24" wheels, bolted on, with steel rims, and the goal was more to organize bikes neatly than it was to secure them. The fact that these racks continue to exist is just a reflection that we still haven't got through to people that bikes are not just toys for kids.
If you reverse your bike so the rear wheel is in the slot, next to the angled bar as everyone else has recommended, and remove your front wheel, you can run a ulock around the seat tube and both rims, nice and snug and secure. That rack will hold four bikes this way.
Of course, messing with the front wheel is easier if you don't have those stupid lawyer lips and maybe full-length fenders are a problem. In which case, switch out the QR skewer for a locking skewer and leave the wheel on.