Single Product Niche Site vs Multiple Product Niche Site

I'm a little behind in performing the assignments due to life happening. At this point I'm still working on my niches and an overall strategy. That's my question - single product niche sites or a niche with multiple products. Let me explain.

Jeremy has shown us several examples of single product sites - like ultramobleguide.com. I understand the marketining, design and significance of a single product site. However, Jeremy keeps stressing that our overall goal should be sites that are designed to last. In that light, let's say I want to do computer equipment for home office users - backups, monitors, PCs, audio, printers, etc. I might even narrow the focus to home office users who are sales professionals or photographers for example.

Questions:

1. Is it better to choose to set up a site for each product category - pcbakcup.com, monitors.com, etc. Or would it be better to select a site domain like homeofficetech.com? If I chose homeofficetech.com, I could start with one product category, build it, market it and develop sales from it. Once sales were coming in, I could then add another product category to the same site. Obviously there would be a home page with competing products but I could still have landing pages for product categories (i.e., PC backups) and for specific recommended products (i.e., low cost backup devices). By doing it this way you would have a site with more content and that might add more value to users.

2. Is the single product niche site really designed for long-term? Jeremy's site may perform well for a while, but what happens when ultramobiles aren't hot? Or they become commonplace? Wouldn't it be better to design sites around a target market rather than a product category?

I'd be interested in Jeremy's thoughts on this as well as everyone else's input.

Thanks!