Use MSBuild to build old VS6 C++ projects

First point of fact is that the solution file pretty much magically becomes a MSBuild file when MSBuild executes it -- which is what happens when you build the solution in visual studio. In addition, all those project files are just msbuild files managed by visual studio. In fact, the project files handle most of the real dirty work here no matter if you are building from solutions or building from a custom msbuild script.

We also use TeamCity to build and publish applications and we will typically end up with custom msbuild scripts for most projects. The role these scripts play -- which isn't really easy with a solution file -- is packaging the project for deployment or distribution. Typically these scripts handle some more operational aspects of things -- like building the web project to a particular folder or copying in the running configurations as opposed to the development configurations. The heavy lifting tends to be handled in the project files themselves -- the build script just references those, we don't try and recreate that wheel. Overall it works great and it isn't really much duplicated code. Typically once we get the build scripts up we rarely have to touch them until there are massive changes in a project mandating massively changing the build.

Both fulfill the same purpose of controlling and automating complex build processes. However, MSBuild is more modern, has more capabilites, and it integrates much (!) better into the build process of the Visual Studio IDE (both use actually the same "input file format" like csproj and sln files). Those project files can actually be edited and maintained (at least,to 98%) by a well structured "project properties" dialog. That is not possible easily with nmake files, because NMake was never designed for this purpose.

IMHO NMake is shipped with Visual Studio because of backwards compatibility, because of better cross platform compatibility, and because it is sometimes better suited for tasks where you have to integrate tools into your build process which are completely outside the VS IDE. The latter can be accomplished with MSBuild, too, but NMake is IMHO often a better fit if you have to create your build configuration manually - because that is what make was designed for.

NMake was Microsoft's implementation of the Unix make command, which has origins back to 1977, according to Wikipedia. Because of that history, there are still lots of projects using makefiles. Especially in the Unix/Linux world, make is still a widely used tool, and a tool like NMake provides better compatibility to the make tools used in that ecosystem.

Couldn't compilation of dependencies be separated by different projects allowing for partial compilation when using MSBuild? That would involve having multiple projects in a solution and then building the solution to build the entire product.

That is the standard approach for any bigger project, nothing really surprising. But on that abstraction level, it is not different from make/NMake.