From experience, sharpening is necessary for printing (for most part, but is dependent on what your image is); it isn't necessarily down to equipment limitation, though the degree of sharpening needed goes down with higher end printers.
Absolutely, You have mentioned all the points but i will just raise the distance importance. For example A3 printers despite offers half dpi* and twice as bigger droplet size seems the same quality as taken from 9600*. This is just because we are viewing the image from twice much distance then A4. At a commerical level, 9600* may will be the highest drop per inches value possible at this point.. which was possible by reducing the indivisual droplet size to 1pl. Now more sharpness and high dpi* is possible if they reduce the droplet size even smaller then 1pl..
As far as the equipment is concerned, it Do and Do not effect depeding on how big we want to print. At 4x6" the effect of having high MP do not stand what so ever, the equipment being 10MP should not as you have mentioned be a problem.
The Canon iP4500 is print on is the first printer I've ever used for photo printing, at 9600dpi and printing on 4x6", I still need to sharpen. I dunno if that's a fact of photo printing or a limitation of the circumstances and hardware.
I have heard some of my friends raising the similar point but for the subjects like water...btw, what are you printing?
or You can always sharpen your images in Adobe and print them.