A single building element, how about a window, will be represented multiple times in the construction documents.
There's the window in a plan, at least one elevation, often two or more sections, maybe/probably an interior elevation, maybe a wall section, and the window schedule. Then there are dimensions and annotations related to the window. Five to fifteen representations of one element is probably typical.
The goal is to generate these representations with as few project elements as possible. The ideal is one. With a one-to-one ratio of project elements to building elements, you can focus on manipulation of an element, knowing the representations will largely take care of themselves.
With multiple project elements per building element, it falls to you to maintain the integrity of each and every representation. You work more, do the same things over and over, have less fun, and make more mistakes, which, considering how hard you've been working, is a downer.
When you choose to draw a building element that could be modeled, you are shifting the responsibility for the representation of that element away from the software and onto yourself. When you unlink a section/elevation, you are signing on to change that one door or window multiple times. As multiple building elements change multiple times, the added work, and the risk of error, grows exponentially.
This ideal of of unity is only partially attainable with current technology. But you should have a good grasp of how attainable it is.
For example, full-height walls are relatively unified. They display well, automatically, in a wide variety of contexts. Low walls, however, are less unified: Since the plan and section fills can't differ, you need two elements, a wall and a slab, for the the plan and the section. (Update: In 10 this is somewhat improved. You can show a top view of a wall, but you can't control the fill.) (Update: In 20, you can use Graphic Overrides to put a background fill color on the top. No fill pattern, just the background, but still an improvement.)
The reflected ceiling plan is a great divider. Many elements need to be drawn over. Beams, dashed in plan, need to drawn solid, while floor elements, solid in plan, need to be drawn dashed. Given the current design of the software, we're stuck with this. Update: Graphic Overrides changed everything.
A lot a higher-level object design is concerned with getting objects to multi-represent themselves better.
Our job as users is to know these limits, do our best within them, and work around them when we can. If you can cut the number of project elements for a building part from 5 to 3, do it. Reduce repetition where it can't be eliminated.
Part of knowing the limits is noticing when they change; note the RCP comment above. It's important to stay informed about developments in technology which can lessen repetition. This includes improvements in software and in our own libraries and standards. This, in turn, means a willingness to change our habits, abandon obsolete workarounds, and adopt better techniques.