From the example given above, you can see 3'b100 is an illegal op code and as per protocol if that value occurs then its an error. So here instead of writing and illegal_bins you can have a assert property with coverage to check specifically this scenario.
It is actually a debatable point because illegal bins will stops simulation if it hits, and from the last message or from debugging engineer can debug the reason for failure. The mail point is as it is not allowing the simulation forward, you can not check the functionality in error case. So to avoid this constraint we have assertions with cover property which allows us to go ahead with simulation with error indication.
So usually I would prefer to have an assertions (with cover property) where strong protocol check requires instead of writing illegal_bins.