I'm actually working on an indie 2d RTS right now, and I had to research 2d vs 3d graphics a little bit, and got to a few conclusions.
Perception-wise RTS games work better in a 2d setting. However they'll have a faster development time and will be easier to sell if they are 3d.
That's the short version. The long version is that 2d objects keep their outlines at all times. 3D objects outlines are ever changing depending on their movement and camera positioning, which can become an issue.
Human perception attempts to shortcut as much of what you can see as possible. If unit's outlines outside your direct focus are changing at all times, your units can become lost in the screen (particularly in the midst of a heated battle). Games such as Warcraft 3 overcome this with some degree of success by having exaggerated features, therefor enhancing an outline that is easier to follow outside your direct focus.
On the other hand, creating isometric 12 sided 2d sprites is very time consuming. For the aforementioned indie RTS I'm slaving my ass on, I created a lot of hand drawn 2d sprites and it takes a lot of time. The end results look a little quirky too (which we hope to get a free pass on since we have no budget).
If a triple A studio were to make a 2d RTS, it would cost them a lot of money. Professional 2d animation costs a few dozen thousands of dollars to create seamless animations (with a lot of the work being finished overseas).
However, 2d units contrast in the screen can be effective since there's a little bit more of direct control of subject and background.
(working screenshot)
So in that image I worked the tiles to look painterly whereas the sprites look like old cartoons. It kind of works.
The workload would had been halved considerably have we had worked these in 3d. However we didn't had the funding, so we did the best we could. If in my crappy indie I want to change something, it'll take a few days of work on my side.
The last point is that games in 3d are a much easier sell than 2d ones. The last major 2d RTS I can remember was Sunage, a game that was
destroyed by reviewers, in large part because it was a mid 2000s game with 1998 graphics (also it wasn't up to modern RTS mechanics). I still think Sunage looks gorgeous though:
On 3d RTS games on the otherhand, you can make the entire thing look far more exciting than what it really is. Even the so so Dawn of War sequel looks amazing in almost every screenshot. Even though the player would have no reason whatsoever to use the camera in such way:
Graphics are just meant for immersion and presentation. They are meant to get players to play and buy the game (because no one plays text based games anymore). In this regard 3d games just get the job done more efficiently from a business point of view. Which is a shame, I don't particularly think they work better for RTS games and I kind of miss the 2d games of the late 90s.
: The basic rules of the tactical combat are fairly simple. It wouldn't be hard to release it as a board game, or a 2D graphics TBS game. I think such a game would actually be pretty fun. So no, TBS games don't necessarily need 3D graphics.
: 3D graphics can definitely add a lot. XCOM: EU would be a far worse game without the quality 3D graphics and exquisite audio work helping to build suspense and set the atmosphere.
: As a consumer, seeing a game without 3D graphics today doesn't tell me they skipped on 3D graphics but spent heavily on developing other parts of the game. It tells me that most likely, they skipped 3D graphics because they couldn't afford them, and the game is a low-budget indie game. I have nothing particularly against indie games, but you do have to be more cautious when buying them because there are some absolute stinkers out there.
