How crucial is 3D terrain in a classic RTS game?
My love of RTS games is from the days of Warcraft II, C&C, etc, when top-down/isometric views were king and terrain was generally tiled and flat. More modern games - as CPU/GPU power increased - moved to fully 3D terrain but I wondered is this considered a necessity in a modern game or would a more old-school design still have mileage? 3D terrain adds tactical elements but are those essential, or typically not that important to the gameplay, or even gimmicky?

3d terrain always comes with a cost: Objects get obscured behind cliffs, so the player needs to rotate the camera to maintain situational awareness. This takes time and disorients the player. They end up fighting the camera more than they fight the enemy.
So before you add height levels just for the heck of it, consider how much it adds to your game in regards to gameplay. When units on higher terrain have an advantage over those on lower terrain, it can add another interesting aspect. But it's an effect which you need to balance. You have to consider for yourself if this fits into your overall game mechanics and if it improves them enough to justify the usability problems it causes.
Also keep in mind that height levels and a 3d engine aren't dependent on each other. 
There were lots of games during the RTS boom in the late 90s / early 2000s which used 2d engines and still used height levels with all the gameplay ramifications. 
And there are RTS games which use 3d engines but make almost no use of height levels. Warcraft 3, for example, has a distinctive "fisheye" look to show off the shiny new 3d engine which sets it apart from its predecessor, but makes very sparingly use of height levels. Most of the gameplay feels 2d. I am not even sure if height even has a gameplay effect in WC3. When I remember correctly, it was a purely cosmetic thing.
