The simple answer for using OSS or proprietary software is - whatever works best for your purposes/needs and whether you can justify the (monetary and non-monetary) costs.
Some software just isn't available as OSS, or what is available isn't nearly as good as the proprietary. For advanced macro usage and the crazy stuff some people make Word and Excel do, OpenOffice/LibreOffice just can't compete.
Let's face it, most people grew up with Microsoft and Microsoft-based products. It's what they know, and the known is always going to win out over the unknown, even if the unknown is superior. This doesn't just go for proprietary vs open source. Many people use Microsoft over Oracle for the same reasons. It would still hold true if your question was reversed and the circumstances were reversed (they were advocating close source and you wanted to use open, and the world ran on open source).
Photoshop and the design industry is a great example of this. Photoshop is the design industry standard. As good as Gimp is/can be for personal use, it still has glaring compatibility issues with Photoshop (layer grouping, anyone?).
It Fits the Workflow
Sometimes, it's just that the proprietary software is better suited to one's needs. For example, if I had to chose between Eclipse and Coda, I'd take Coda any day, because it suits my needs and workflow better.
It Supports A Good Company
Assuming the tool your using is getting put out by a company that meshes with your views on business ethics or whathaveyou, the best way to support a company is to use their product. A lot of the OSS fanatics use only OSS because "software companies looking to make money from their product are evil," despite the fact that if they're a developer by trade, they're doing the exact same thing. Companies can be good and still sell their software.
All in all, I'd say if you haven't tried the title they're offering as an alternative to your chosen piece, go ahead and try it in your spare time, if only to humor them. You can then determine whether it fits your workflow and can better tell them "I just don't like the tool you like."