Jules way of expressing his opinions leave a lot to be desired. But
his basic assumption is a correct one.
Like I argued earlier in this thread, image quality is the result
of many, many small and large factors. The sensor itself, and the
data processing that is done after capturing an image play a big
role. But there is more to the equation then that. Way, way more.
When I shoot sport I mainly rely on timing. But occasionally, you
just have no clue what is going to happen next, or exactly when.
Then your only option is to fire of a burst of images and just hope
to get whatever interesting moment that fly by. Try that shooting
raw with a D50 (or a D70 for that matter). You get four images,
then the camera locks up for several crucial seconds while writing
to the card. And that is when the goalkeeper make that amazing
save. Or the outfielder does an insanely great catch. With a D200
(or the D2H that I use) I can fire of long (5-10 images) bursts at
5 (or 8) fps. Re-aim, fire of another similar burst, and basically
keep doing that until the action has stopped.
When you work with photography, you live with your camera. It is
always with you. In rain, in snow, when taking a fall down a ditch.
Put a 300/f2.8 on a D50 and make a nose dive to the ground. You
will very likely break the lens mount, because the D50 (or D70 and
D80) lens mounts are not built to take such abuse with a five pound
lens mounted. The D200, D2H(s) and D2X(s) are built to take just
that and continue working. How good is the IQ on a camera with a
broken (or semi-broken) lens mount?
Or, after a maybe a hundred thousand of accutations, the shutter on
a D50 (or D70) will probably become very irratic and unreliable. On
a pro model will happily click on for years to come. How good is
IQ when the shutter does not work properly?
But let's focus on pure image quality for a moment. You seriously
think there is no advantage at all having 10 megapixels? Ok, I know
a lot of people over estimate the importance of megapixels (as a
D2H owner I am fairly used to discuss that issue

. But again,
occasionally you get a really good image -- but with a wide angle
lens on, when a longer lens would have been the best option. With
10 (or 12) megapixels you can do a lot more cropping and still have
image resolution to play with. The difference is maybe not huge,
but it sure is there.
Or let's look at the very, very good image processing the D50 does
when shooting jpeg. That is great. But it does mean you leave a lot
of your post processing desicions to some unknown Nikon engineer.
And people that live of their camera tend to want to make those
desicions themselves. You want control. You want to get the image
data as unprocessed as possible out of the camera and do things
your way. The D50 has som great built-in image processing, but that
is of little value for someone that want to stay in control of the
workflow.
Or take a simple thing as working with manual exposure (yes, a lot
of people still do that, with good results) -- that's when the
D70/D80 shine over a D50 wth the extra command wheel. A simple
thing like being able to quickly adjust both aperture and shutter
without extra fiddling around is a big benefit in many situations.
Again, it is about that extra option some people want and/or need.
It cost a little extra, but in the end it is a way to get the image
quailty you want.
Sp the D70 have some slight advantages over the D50 in terms of
working manually, and some actually people prefer they way its
handles image data lifted from the sensor compared to how the D50
does it. The D80 adds more pixels to that. The pro cameras is
litteraly in another league: They add durability, pure speed,
faster handling (more buttons, less menu fiddling), more accurate
(even in dim light) AF, more choices in terms of image processing
and lots, lots more.
So the D50 sucks then? No, it is a great camera -- if you use it
the way a D50 is intended to be used. It is not built for pro use.
It is not intended for those of us that shoot mostly manual mode.
But from what I have seen it is (together with the D2Hs) the best
noise performer at high ISO in the Nikon crowd. It has an excellent
jpeg processing (I hope the D80 gets a piece of that), and it is
hard to beat for image quality/dollar value. But to argue it gives
the best image quality over all ... Get real