MMDirectX is a Merge Module for DirectXTM v8.1, which you can merge with your MSI setups. MMDirectX allows you to build a dedicated installation for deploying DirectXTM or include DirectXTM as a part of a more complex installation.
The main advantage of MMDirectX, besides the fact that it is an MSI wrapper for original Microsoft installation of DirectXTM, is that it includes different setups for Windows 2000 and for Windows 98/ME. During installation phase MMDirectX detects Windows version and automatically selects appropriate setup.
Source: Contributing Member, Leonid Sadofiev