I definitely agree that with some builds, getting a pre-made piece of terrain is almost necessary.
Obviously, it saves time (always remember: real life, jobs, etc.; not everyone here is in their teens, with tons of free time
). If you really need to test a builder's terraforming skills, do just that, but don't make people spend the whole week just getting the full plot of terrain right by hand, when large-scale terraforming is in practice done with other tools, like WE.
Also, terrain both guides you and limits you in a good way. It makes you work within some bounds, and you can incorporate some random features into your build, so they suddenly make sense together, and don't look as artificial as they would if you designed both of them at the same time. When you can (and have to) build the whole terrain yourself, you're tempted to "cheat" and make a piece of terrain that just happens to suit perfectly to your needs, so you don't need to be clever with the buildings. That is okay for smaller-scale terraforming like adding a rock pillar, but a full-plot valley that curves exactly the way you need... that doesn't provide any external inspiration, and takes away most of the time, while not really helping you improve the important skills...
It doesn't have to be custom-made for the theme, it can be just a copy-pasted cube of an appropriate area of the main world -- for instance, for the current Dale, a cube of the Misty or Blue Mountains with a river would be great. (It doesn't need to go exactly according to the lore every time, such as having a precise bend in the river.)