A question that arises out of this is, should the 90% use cases be handled by a small, simple tool, or by the tip of the iceberg of a large, complex tool such as curl?

I can see the advantages of standardizing on a complex, powerful tool for simple use cases. For one thing, it may be the only way to standardize: simple versions are too easy to write, and therefore you get dozens of competitors, none of whom are popular enough to shake out their edge case bugs.

It's also nice not to have to find, install, and learn a new tool when you stray over that 90% boundary.

With software dependencies, I think the advantages of small, simple libraries win out over generality. Supporting powerful use cases makes an API more complex and often means that simple, 90% use cases require weird incantations to make them work. Here I think of the times I've had to get into the guts of Jackson despite never doing anything remotely exotic with JSON. The 95% of your codebase that can be simple should be simple, so you can devote your attention to the things that need to be complex.