I personally prefer steel. I broke a set of plastics awhile back and I have not yet broken steel, though those are just my anecdotes. I find steel a bit easier to get into as well, though the narrow steel clips aren't as comfy for me.

So, no, plastic isn't bad, but different people prefer different things.

I would have to say look out for the cheaper plastic toe clips. The more poorly designed ones can sometimes catch your shoe laces which has the same affect of not knowing how to unhook from clips, leading you to face plant into a shop window and ultimately laying on the ground, your bike on top of you with an entire store looking at you and your big scuff mark on your nose and forhead, much like a rug burn, that takes about two weeks to heal... ****ing hate plastic clips.

That and I think they don't look as stylish, but if we all followed that train of thought we would have a bunch of people riding around the streets a high speeds not wearing any type of head protection and that would just be crazy.