I'm sorry, but if I were a concept artist (alone), I would not choose to work at a place that imposed hardware limitations on my imagination. A concept artist's job isn't to figure out the polygon count, and reduce their pen strokes so that the visual can be translated without issue. If that concept artist is limited by hardware, then someone's doing their job wrong.

Obviously, the default task could be altered by limiting factors - like a reduced development team where a concept artist has to double as a programmer or graphic artist or some such. But a concept artist? No final-hardware-based limitation on creativity. Often concept artists are even starting their work before anything is known about the final product (same for movies as well)

: Looking at the Halo Wars concept art, particularly the sprawling
: battlefields, what is depicted was/could not be translated into the game.
: It's pretty, but maybe absolutely ambitious concept art isn't helpful to
: the design team.

I'd say yes, it would be helpful. There are "artists" throughout the development cycle. Ideally, the concept artist should be creatively freed from shackles, and further down the line their vision is optimized and realized by other artists whose specialty is graphic design and/or programming (or perhaps shed altogether and redesigned apart from the concept artist, who knows)

Of course the concept artist can work with anyone else on the team to 'see' what the ideal environment could look like. And of course there's no reason that all concept art must reflect in any way the final product. It's ultimately a guide, and an inspiration, to graphic artists and programmers; something from which to work and/or to work towards.

: So programmers, the people who work behind the scenes to make a game
: function, broke into the art team's cubicles one night and uploaded a
: bunch of overdesigned models, just so they could show off their software?
: Their programming skilz?

Um... no?
I don't know who decided to put the "garbage" into the game, do you? The artists? The programmers? TPTB? Whoever did it, if it wasn't part of the initial vision, then the initial comment about this 'issue' is true - they dropped it in just because the engine could handle it. Whether it was artists further down the line, or programmers who felt they could just increase the asset count. I don't know. Someone just raised the issue of 'garbage' in the game they felt was needless and basically clutter for show.

:: Each stage in the process should have an expected output standard.

: Which is 180 degrees from "Do not limit the artists who design and
: create the Halo vision". A correct way to handle a project, yes, but
: still the complete opposite of what you stated earlier.

Let's go back to what I said above.
Concept Artist != Graphic Artist != Programmer. There may be more roles in a development team, there may be fewer. I'm just throwing out positions for the sake of this point. Concept Artist - no limitation on imagination. Graphic Artist - translate or be inspired by concepts to create digital content, optimized to work within the known bounds of the hardware and software's capability. Programmer - Make It All Work. Well.
But if you're arguing semantics on that first one - "no limitation" is an "output standard". The standard is: go nuts, be creative, be imaginative; we'll whittle it down as we find out what our game can handle. Or bring it back towards the vision if we find there's room to improve at any point.

: Still blaming the ambitious programmers?

If the programmers were not tasked with creating the visual esthetic, and they 'messed' with an accepted design, then I wouldn't blame the artist for being upset. nonononono, that's not how it was supposed to look! IF it was programmers who dropped the 'garbage' into the game just because they could, then there may well be artists shaking their head. IF it was any artist along the line, then who knows. Everyone on the team may have thought the 'garbage' was a good addition, but it ended up being the players and community that didn't like it. Hey, that happens. :P

When it comes to Halo, there are segments of the community that love the visual design, and there are segments that hate it (and segments that still long for the simplicity of original Halo esthetic). That's why I was curious about the original Halo environment concept art, to see how detailed the visions were, as an indication of whether the initial artists envisioned simple environments from the start, if the artists may have been told to limit the complexity in their art, or if detailed designs were whittled down later for hardware reasons.

Looks like many of the designs though are relatively simplistic (reflective of the final product), and so either the initial vision was that eg Forerunner style IS geometrically simplistic (and more recent 'busier' designs have changed that original intent), or the artists were informed to not be too complex due to hardware limitations (and more recent 'busier' designs may be in line with what initial style visions implied, but weren't 'allowed' to depict). *shrug*

At least they didn't resort to matte painting use if the concept art is too complex to implement :P (though, skyboxes could really be considered a form of matte painting; or like Doom's sky).

One could look at the way 343 did Halo 4 levels, and the lack of theater, as a result of attempting to realize more of the Halo 4 artistic vision/detail (love it or hate it), at the expense of freedom to explore. Of course, a controversial design choice.