Some arguments just aren't well-developed.
Some arguments are cluttered with unnecessary additions.
A good argument is a thing of beauty, with elegance, subtlety, and often great complexity and detail.
And for an academic work or essay, nothing is more important. Without an argument, there's no paper. Without a good argument, there's a piece begging to be ignored. With a good argument, people learn. Or at least I learn.
I see an incoherent mess, and I'm pretty much ready to dismiss it.
I see a brief, well-thought out piece and my interest is held, and I wonder about how the ideas develop.
If you have a good argument, it's not a bad plan to try to present it in as brief a form as possible. Such an attempt aids one in any ways: it's usually quicker; it's easier to avoid entanglements; and it's the best form to get feedback.
It's much easier to think of ways to fill out a sparse argument than it is to disentangle competing threads of a confusing argument. On a practical level, it's easier to give feedback on a shorter piece than a longer piece, if for no reason other than that the longer piece takes longer to read.
A good argument requires focus and consistency. It has to build on itself; it has to minimize internal contradictions; it has to recognize and acknowledge those that exist. It had best acknowledge, in particular, points that would be widely debated.
You can make an argument that the sky was yellow and the sun was blue if you want, but you need to acknowledge that there is general consensus to the contrary. An argument need not accept common knowledge. It must, however, develop the argument strongly; it must avoid mistakes. If you want to show that the sky is yellow and the sun is blue, it doesn't help your case if you insist that the whole world is in black and white.
What is really a shame, I think, is when a writer a sound argument and then ruins its strength and beauty by adding unnecessary details and arguments, especially if those unnecessary arguments tend to contradict other assertions, or are somehow grossly in error.
Of course the good argument is only one aspect of a written work. In addition to working out the logical aspects and logical relationships, it is also necessary to work out a plan of presenting that information. Logical structure is not necessarily linear. The written form is. (Even hypertexts that can have largely non-linear structure have some degree of linearity--every sentence is linear; so is our experience of time.) But that's another essay.