I agree - but what is the purpose of the rubber plug?

The purpose of the rubber plug is so you can get a grease gun on the fitting.
You pull the plug out, stick the grease gun on the fitting and pump in new
grease as the old grease is forced out the hole where the plug was. When clean
grease starts coming out, then all the old grease has been flushed out. If your
plugs are split or torn, then you need to replace them to keep water out and to
prevent grease from slinging all over your wheels when you go down the road.
Mark McCoybr
McCoy's Market Bumpus Mills, Tennesseebr
http://www.mccoysmarket.com