D3 files are pretty much the same size as D2x files. And frankly, a D4's files aren't all that much bigger either.
High ISO - well, some of us shoot in conditions that demand it. I shoot a lot of minor league baseball - where ISO 3200 is marginal even with f/2.8 glass. If you don't shoot at ISO 6400, you get blur. And if you do shoot at ISO 3200 on a D2x (which is Hi-2, not 3200), you get lousy contrast and a lot of post processing work to remove color noise. Sure, I've been known to process nice clean files so that they look like I souped Tri-X in AcuFine, but that's an artistic choice, not a requirement. Even as a fan at MLB, most of the parks don't permit big glass. So often I'm at a night game under good TV lights - with an f/8 lens. Not such a great combo with a camera limited to ISO 800 - been there, got that T-shirt. Even a day game is no picnic if it isn't a clear sunny day.
I have a lot of images on my disks. If you're shooting a motor race and you need to have more than one of each entrant, and there are 200 entries, you end up with a lot of shots. Twenty per entry yields 4000 frames in a weekend, and between the various locations on the track and the necessity to be sure to get at least a couple of good ones, it's easy to use 20. If you're shooting a playoff baseball game, one might end up with a lot of shots that are wasted - because you won't know until it's too late that he's going to hit the game winning homer - or strike out. Or if you're working on panning, better shoot a bunch the slower your shutter speed goes, because that's a low probability shot. Shoot 15 to get one good one- maybe. I'm not talking about panning at 1/125th. I'm talking about panning at 1/15th. Maybe even with a 400mm lens. OK, 1/30th with the 400. 1/15th with the 80-200. We didn't do stuff like that back in film days - it was just too expensive.
Why would I want more than a D2h? Better AF is one reason. It's not lack of skill when a D2 doesn't catch IndyCars at 230mph, especially when a D3 catches them every time, even with a lesser lens. Another reason is LiveView. I use it a lot, for things like macro and architecture, which definitely are NOT the type that generate 9 fps. More like 9 frames per hour, or actually maybe less. Assuming we're not talking about stacking, panorama or HDR, all of which chew up shutter cycles quite quickly. In fact, stacking, pano and HDR are used in landscape and fine art photography, both disciplines in which one is not generally prone to the "11 fps syndrome." Yet I remember more than once consuming 20-40 shutter cycles on a single composition - "one frame." As an example, a 4x5 panorama. Or a 6-wide pano, each frame of which is a 3-frame HDR. Or one model railroad shot consuming about 14" of DOF - which translates into 33 frames stacked. Four of those and you're up over 120 shutter cycles - or 1.6GB - even if you don't make a single mistake.
I started photography 40 years ago, in the F2 era. I know that one thing I am far more willing to do now is to work a subject. If each frame costs $0.20 (bulk loaded film, self-processed E6 or B&W negative film, and a contact sheet), it's easy for an amateur to be satisfied with one or two shots, even if there's a niggling thing in the back of your head saying "not really exactly right." Now when even retaining five copies (mirrored primary system, mirrored secondary server, off-site disaster spare) costs under $0.005, or approaching two orders of magnitude less, I'll keep working a relatively static subject like a flower, possibly for a half an hour. Yet I don't think this falls into the "just start cranking off frames." It's still careful, thoughtful work. There's just more of it. I actually work on the craft of photography a lot harder now that it's "more affordable." If the #23 Lotus 51 didn't get captured 100% perfectly last lap, I'll shoot it again this lap to be sure that I have a killer crisp one this time. I'll shoot quite a few practice shots, to refine skills used "in the real event." If anything, I think I'd say that I'm a lot less lazy now than when I was shooting film.
At the same time, I'll certainly admit to being tempted by the D800e. And yet, realistically, I've had less than a dozen situations in which the additional 24mp would have gained me very much over the 12mp in my D3 and D2x. Which is one reason that I still have a D3 and a D2x, and not a D4 and a D800e.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!