Why no warning for unused let bindings?

For those of us that don't have Visual Studio and edit the fsproj by hand, the way to implement Tomas's answer is

<PropertyGroup>
    <OtherFlags>$(OtherFlags) --warnon:1182</OtherFlags>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>

As an example, see the project file of FSharp.Core itself.

You can enable warning for unused bindings via the warnon compiler option. If you want to be strict, you can even use warnaserror+ to turn it into an error.

The warning number is 1182 and it is turned off by default as documented in the compiler options page in the F# documentation.

fsc --warnaserror+:1182 --warnon:1182 Program.fs

How to do this will depend on your editor. In Visual Studio, you can do this by specifying "Other flags" and "Treat warnings as errors" in project properties.