Interesting thread.
I think the answer to some of your questions is that blizzard cliffs serve a different artistic style than custom made cliffs.
WC3 art style is 'cartoonish'. This means very bright colors and strong contrasts, simple shading with an almost cel-shading anime-style look on units.
It's not that those cliffs are ugly. In fact, you can make some pretty good terrain with them - I did that in the past (well, actually, this was 8 years ago, when I started mapping for WC3) and was pretty pleased with the results. The trick was to use cliffs in combination with terrain height modifiers and doodads.
About the blizzard trees, they serve their purpose perfectly: create a colorful, cartoonish environment which looks great from birdseye view.
Note that wc3 is an RTS; all of the models look great from above, whereas the side views are almost always not as polished, as polycount was a real issue in the days when the game was new - the ingame water is a nice example for this ... the GPU consumption is ridicolously cheap (its just one plane with an alpha texture) and it looks good from above, but not from lower angles.
With raising map quality over the years, people wanted to achieve different art styles within the game to support other game styles. This was when the Sci-fi models and highress models came up.
It's not that blizzard trees or cliffs are ugly, they just don't fit if you don't want the cartoonish style of WC3.
Note that some of the most popular maps in WC3 history embraced the cartoonish art style of WC3 and allowed for custom models that match the style to shine. DOTA is the best example here.