Ahhhh...well, then...I hope you don't use a dSLR when you need reach, because it sounds like the Canon PowerShot SX30 IS is the perfect camera for that.Why are you talking about PS when I'm talking about DSLR?
Perhaps I should let neuroanatomist reply that one to you, but since you don't seem to appreciates each others argument, maybe I can help:
1) You said there are only
advantages to having higher pixel densities. That is obviously an exaggeration that you probably did not mean literally.
2) neuroanatomist gave the example of SX30 to show that a higher pixel density is not sufficient. He probably gave the example to make you think and realise why you are wrong in the "high pixel densities always rule" assumption. However, I don't think his example was very successful for that purpose because a) There are many other variables in addition to pixel density that are not kept constant (like sensor size, optics), so it's hard to from this one example to disentangle what the significance of the higher pixel density is. I tried to resolve it in a post above, but perhaps it was too technical to read well. b) He was not very clear with what the example was supposed to demonstrate. c) The tone in his reply was unnecessarily deprecatory, bound to fail an explanatory purpose and trigger the reaction you gave.
To be clear, here is an explicit list of some disadvantages with higher pixel densities (in approximate order of significance):
a) Readout noise increases with number of pixels
b) Slower readout time (limits your images per second rate)
c) More quickly gets limited by diffraction, so needs faster optics to be useful
d) Requires proportionally smaller tolerances for the camera house / optics to make use of the pixels
e) More sensitive to illumination direction (limb darkening)
f) Space/processing requirements increase
g) More expensive to manufacture
Note that I don't list noise
as a disadvantage, since the quantum efficiency and collecting area does not change much with pixel density (only in the case where you are read-out noise limited will higher pixel densities produce a noisier image). Also, the dynamic range
will not change significantly either, because the storage capacity of pixels is usually determined by their areas, so even if pixels are smaller and have smaller capacities, the number of photons they have to take care of is proportionally smaller.
The list of advantages with higher pixel density I can think of is much shorter, but very significant:
a) Potentially resolves finer detail in an image
I say potentially, because this statement is only true within certain limits. E.g., there is a limit how fine detail the optics will resolve. For small apertures, this limit may well be the diffraction limit (this is a physical limit); in general it is probably more common with imperfect optics. It also assumes that you have sufficient light and short enough exposure time for the pixels to be well exposed without introducing motion blur at the pixel scale.
There is always a balance between the advantages and disadvantages that sets the optimum pixel density. Depending on how much weight you give the different properties, this optimum balance will shift. For FF cameras with current optics, I think a practical upper limit on the number of pixels is around 50 Mpix, approximately the pixel density of 7D. Going beyond that does not make much sense to me, unless there is a revolution in lens manufacturing. I expect higher resolution images will be the domain of larger size sensors, medium or large format, as it is much
easier to produce appropriate optics for them. I believe this will be even more true in the future, as sensors will be increasingly better and less expensive, while I don't expect manufacturing of optics to improve at the same pace.
I'll be happy to discuss any items above you may disagree with.