It's 2015 now. Why do DSLR cameras still use monochrome LCD as their top displays? A raster graphics display screen would be more useful and could display more different information.
The main reason is that it is low powered. An raster LCD - color or not - requires much more power and most of them need a permanently on back-light. New OLED display do not need the back-light but still pull more power than a segment-based LCD which has much fewer electronic circuits.
Most current top-plate LCDs mirror what is shown in the status line of the viewfinder, plus a few more settings which allow those to be set while looking at the camera from above. Each of these settings have a few states, so they do not require some that needs do be drawn from pixels.
It is only a matter of time, I would guess, because there are very useful things one can add. A color live-histogram would be great, or focus information, particularly for tracking. Now, anything that is possible is most likely to be implemented first on the rear color LCD. For this reason, I think that some manufacturers are likely to implement this on their high-end models to distinguish them as more advanced, almost just because they can.
Indeed Samsung did that on their Pro815 Ultra-Zoom and the top display not only showed quite a lot of information, it also serves as a waist-level viewfinder. Live-view on the top-plate, essentially.
Segment LCD is a technology with the lowest power consumption of all available display technologies and those are more than visible on sunlight or any kind of strong light without any need for background lightning (except at night). Very useful for battery operated devices. It's also the cheapest possible display technology. They are also extremely robust. Remember those cheap digital wristwatches? The battery is extra small in them and they still manage to keep going for a year or two...
A few words from one of many LCD manufacturers:
Segment displays have the lowest power consumption of any of the six technologies, which makes them a good option for battery or solar powered products.
They require the lowest power to drive, an estimated 2uA per Centimeter squared; in fact, glass only displays require an estimated 10% of the power that is required for a LED backlight. In other words, a segment display without a backlight will draw around 1mA; the same display with a LED backlight will demand 10mA up to 25mA.
If the back screen tilts up, you can use it from above too.
I note that the old fashioned LCD on the top screen is readable in sunlight. It uses a backlight to read in the dark, but is read using ambient reflected light in daylight. The LCD screen is unreadable outside even in moderate sun.
I further note that the backlight has "regressed" from an even electroluminescent background to a crude LED to one edge and my newer Canon cameras are not that great in the dark for this top screen.
So, the top screen is for normal room light and brighter, where passive surface reading is possible, and the LCD is for darkness.
I think a compelling feature for the manufacturer is to have a "low battery" indicator that works when the main system cannot be powered up. A spent battery, presumably just too low to work properly, will cause the batt icon to flash rapidly.
A quick answer to "why did my camera just die?" is a good "pro" feature.
Those LCD are probably already displaying every information you want/need, and probably more. Mine (EOS 7D) has :
- Shutter speed
- White balance options
- Battery charge
- JPG and RAW size
- Metering mode
- Focus mode
- Number of pictures left
I doubt displaying more would be any help and have a proof of it : even the main display won't show you much more... The only think I could think of is a live histogram, but even that seems to be a gadget.
Moreover, I think that a monochrome display is perfect when the sun is out, good contrast and it won't drain on the battery.
You could do the same remark about what you can see in the viewfinder. Too much will get you distracted, unless you have a fighter pilot training :)
One aspect that was not mentioned regarding display power consumption: You can also serve a segment based display with something to display using a much smaller, much slower, and much less power demanding auxiliary CPU.
Segment LCD and a few buttons? You could use an 8051-like core driven from a 32KHz watch crystal to usefully interact. Quarter-VGA graphics display? Font rendering alone will make a snappy user interface difficult to do on the same CPU at 4MHz. The faster you clock a CMOS microcomputer of any kind, the more power you use.
A raster graphics display screen
I think you have misused this word a bit. You see, all the displays are raster - they make use of pixels to form an image. In fact, there is no such thing as a vector graphics screen (as far as I know). Some holographic stuff could be the closest thing we have to a REAL vector image (for now we display vector images on raster screens).
So, back to the main question - they use those little b&w screens because of readability and low power consumption. They mainly have a purely informative role of displaying key values to the user, you wouldn't view any pictures on them anyway.