First of all,
I think the confusion with this is the word "trim". It sounds optional, but it really isn't. It is sort of like getting leather seats or cloth seats in a car. You can choose which you want, but you don't get to delete seats altogether (and this case you have to pay for it separately).
The kit for the bags serves two purposes, one functional and one that is more optional (but still recommended).
The functional thing is that it holds one edge of the rear reflector. I guess that you could figure out a way to hold it down without the trim piece, but the trim kit is the designed way to do it.
The second purpose is more of an aesthetic thing. It completes the lines of the bag. Technically it's optional, but the bag really won't look right without it. Whether you want it matte or shiny, black or silver, you're going to need the trim kit.
I hope that helps. Once you see the kit you'll realize that it's not really an option whether to get it or not. It's just an option as to which one you get.
I think the confusion comes from the word "trim". It sounds optional, but it really is intended to be part of the bag. I think they set it up as a separate item to give the possibility to semi-customize the bags. Regardless of the intended style, you need one of the kits.
One correction to what versysred said, though. The kit is not available in red. You have to buy black or silver and paint to match your bike. It sucks, but it's true.