I have tended to stick with single vendor solution where possible. The main reason for me is support, dependability, and DR. If there is an issue with other software, it has been a problem for me in the past. If I have to install something else on a server to have my application working, it is a potential issue. If it requires a license key, then it's an even bigger deal.
In the past I've avoided Perl scripts on Windows servers because it's one more thing to install and not widely used. Instead I've used VBScript. Is it less powerful? Sure. More of a hassle to work with, absolutely. However I can get the job done with VBScript, so I've used it.
However there are places where software makes a substantial improvement in the services. Generating PDFs in the past was cumbersome. It could be done by hand, or you could install software to help. I think it was worth installing the software.
Backup compression software utilities were worth the improvement in space usage and backup time. Even though this could potentially be a DR install, I think it was worth the improvement.
I think there are valid reasons to stick with solutions from one vendor. If nothing else it simplifies your environment, but make a judgment call. If there is good value in using another product in your environment, why not use it? Most companies won't abandon you in support, especially Microsoft, just because there is another product in the mix.