I'll say it---they probably use black rubber because it looks more interesting than Batman's comic costume. The best elements about the comic design, IMO, are the cowl, the scalloped cape, the gauntlets on the arm, the utility belt, and the logo. That's it. The trunks don't make the costume, and neither do gray tights. Those work well in comics and in (simpler-styled)animation because they make tracking Batman's movements that much easier, through contrast. As long as you have those iconic elements, you pretty much have Batman.
Other than trying to appease fanboys with an incredible eye for picking up details, there is absolutely no reason for Christian Bale to run around in gray tights and black underwear.
Batman is "real" in live action, and a "realistic" approach is a nice way to go at this franchise, because Batman has no special powers. He's just a dude in a suit who has gadgets and knows how to fight. Part of the charm of Nolan's take on Batman is that you get to sense what Batman would be like in our world. Sure, that's not everyone's cup of tea, but it's not a totally invalid way to approach this character.
Those Batman and Robin suits listed earlier in this discussion are decent for fanboy work, but in the end, really look no better than what the best can do at Comic-Con. They look more like costumes than something functional, IMO.
