"Why would you buy a standalone convection oven, when you could combine it with a microwave and get the best of both worlds?"
At one point, I think the Farberware was the only counter top convection oven choice. Although, yes, later on, yes, a real argument could be made in favor of a combo simply to save space. Although, I wouldn't be surprised if even in the early days of combos, people with one counter top oven who were looking to add a different function might have seriously considered buying a second oven, rather than completely replacing the existing oven. As I said earlier this thread, we had a Farberware convection oven growing up. We never had a microwave, but at one point my mother briefly considered getting one. Even if combos existed by that point, I'm sure she'd have opted to keep the Farberware, and add a new microwave. Reason? The microwave would have been the cheapest way to upgrade. To get convection would have required moving up too far in the microwave line. (My parents were terrible consumers. We always had cheap cars, small TV sets, etc.)
Of course among many here at AW.org there is an attitude of: "There is no such as too many appliances!"
Personally, I can easily see buying both. I am very cheap. Buying a new microwave/convection combo is likely to run more than I'm interested in spending--particularly given how long appliances don't last these days. Plus, my microwave needs are pretty simple--all I have ever used them for is reheating. Buying a combo oven used might be hard--it seems like most microwaves I see in the US are plain microwaves. Thus, a Farberware (for convection baking), and the cheapest possible microwave makes sense to me. But that's only for my particular brand of warped thinking.