I personally think beater bars dont give any better performance if you mean a beater bar on a brush bar. I aways think they flatten the pile of the carpet where as the brushes groom and lift the carpet pile getting more out!
I really believe that is purely subjective; to me it depends on what you're cleaning. There were many instances where I thought my Hoover 62 gave better performance than my Dyson DC21 or Kirby Ult. G, and vice-versa...I've found it depends on the pile type. I've used beater bar machines on plush before, and they always did good job of lifting the nap and leaving visible marks. However, on berber/low-pile I've found that an aggressive brushroll does a better job of loosening ground-in dirt. I've also found that on high-pile carpets (frieze, shag, etc.) a more gentle brushroll (without stiffeners) works better. Here I have mostly bare floors which warrant canister use, with the exception of some rugs and low-pile carpeting in one room which I use an upright or power nozzle on. The beater bar is the first choice for the rugs simply because it lifts it up during the beating action, yet at the same time the brushroll seems better at tackling pet hair.
What's fascinating to me, though, is comparing the action of different beater-bar equipped machines. My 700 Special's action is a very gentle tapping, whereas my Kenmore Powermate or 1400-series Eurekas have more of a loud vibration when it comes into contact with the rug. With the 700, it was so gentle I thought I had the height adjusted wrong until I double-checked it, however, it proved just as effective as the more aggressive Eureka. I also agree with what Mike said about the plastic beater bars possibly damaging the carpet after being nicked. I was putting together one of my Eurekas that arrived when I turned it over, and noticed quite a few chips there. It amazes me what people do to their cleaners; that's never happened to me since I've always been careful of what I pick up. A little fine-grit sandpaper and elbow grease, though, and it was good as new again.
I've grown to like the brushroll a lot, but I still wish the beater bar was more prevalent on new machines...seems everyone who previously had beater bar machines is going to a 4-row brush these days, although at least the VGI/II-equipped Sanitaires have them. I'd love to adapt a beater bar P/N to my DC21, it would probably do an even better job than it does now.
