I observe that when using project.json for packages that include MSBuild imports, a [projectname].nugget.targets file is created. This file hard-codes the path to my home directory:

<PropertyGroup Condition="'$(NuGetPackageRoot)' == ''">
  <NuGetPackageRoot>C:\Users\andrew\.nuget\packages\</NuGetPackageRoot>
</PropertyGroup>

This makes the project now unshareable with others on the same machine. If instead, NuGet Restore created the file using the $(UserProfile) macro:

<PropertyGroup Condition="'$(NuGetPackageRoot)' == ''">
  <NuGetPackageRoot>$(UserProfile)\.nuget\packages\</NuGetPackageRoot>
</PropertyGroup>

Then it would work appropriately for all users of the machine.