I did think that for years and I scoffed at multi-monitor setups thinking they were a poor imitation of the virtual desktops that have been present on *nix systems forever. But they are both good and complement each other.

I use virtual desktops as more to separate unrelated areas of work (like different projects). It's like having a completely different desk with different books open on it. Having more screen space is just like having a bigger desk. It's like having multiple books open on your desk at once as opposed to picking up one at a time to look at it.

One big monitor would be ideal (it could be split into an odd number of frames, for example), but in practice multiple monitors is a good compromise between flexibility and price.