That harness distribute's the weight across both your shoulder's so you can concentrate better on the blade. The handlebars are so you can control that blade. The machine can ruin your day quick. The handlebar's and harness are totally necessary.
I wouldn't use the bars because I have done it without and know what I'm doing, however if you don't feel comfortable using it without the bars than use them.
Try putting a blade on and get a feel for it without turning it on and without the handle bars. If you think it feels safe then go for it. also try it with the bars (if you have them already), if it feels safe and comfortable, then you know what to do.
I feel like the handle bars would make it more awkward. I take off the guard above the weed eat head as well. I don't know how anyone can weedeat with that thing on. I like using long string lengths and that guard won't let the string line out too much.
It's trial and error. You know the 'gator boys' catch and wrestle alligators but they don't recommend that you do it.
I have done a lot of brush cutting w/ my Redmax. I don't use bicycle bars or a harness and have had no problems. However I do see how someone could get hurt using it that way if they were being careless. I have done it long enough that I know what to expect. I assume your a newbie or you wouldn't be asking so I think you should do what the manufacturer recommends.
I have a FS90 and have used it many time to cut brush. Usually it's on a hillside where I have to use one hand to help me climb or use one hand get things out of the way. I recomend the harness. Your arms can get tired if your cutting for hours at a time. If your cutting on flat land the bicycle bars can make it more comfortable. Hope this helps.