Perhaps I'm just a dodo, but both from the practice of watching games on the field, and watching them on TV, I find the white knickers much more visible than these black pants. Sorry, but I just don't get the "fashion" related advantages/disadvantages of one style of pants over the other, and I absolutely do not believe anyone on the planet, aside from the officials on whatever field you may be looking at, could possibly care less, either way.
The long pants do offer advantages related to layering in cold weather, and I suspect those advantages would apply if the the pants were white, without without a black stripe. I would like to see a tapered, or even elasticed cuff on the pants, so when they ride down, from all the running around, we're not stepping on the cuffs with the heel.
Remember, the primary purpose of the uniform is to differentiate us from the players, and recognition and visibility are the main concerns. I've got 2 (full uniform) pictures of crews I've worked with; one from years ago where we're all in knickers and one from this season in black pants. Side by side the difference in visibility is startling. These full length pants do offer some distinct advantages, but I wouldn't suggest throwing any knickers away just yet. Fads come and go, traditional standards last a lot longer.