If you are a lurker then you have read many of the threads on this site. There are obvious uses for single handed and two handed rods. Switch rods were intended to breech the gap in certain situations. Two handed casting is enjoyable, less stress on your shoulders, elbows, and back, and a switch rod can take the place of a couple of different sizes/types of rods. The "quiver" Mumbles speaks of is the number of different sizes of rods a person owns or aspires to own. If you really look at fly rods, they are far smaller in diameter than gear rods, for the most part and every weight, brand, lenght and action casts differently and is used in varying situations. Most gear rods will cover a number of situatiuons and are, by nature, heavier than fly rods. No one fly rod will cover all situations and that's why it's necessary to more closely define the situations you will be in before true recommendations can be made.
Once you decide what you are going to chase then you can buiild your quiver but a single switch rod will not be sufficient for all situations. They work well, as suggested, in tight spots and they cast farther than single handed rods while overhead casting, given the skill level of the caster is consistent. Personally, I'm moving away from single hand rods because of the stress on my body but I will not be getting rid of all single handed rods because they have specific uses for types of fishing I like to do.
But... if you are river fishing for salmon/steelhead only and you have the ability to "pick up" the skill, then two handed will work well and a switch rod will work. I've been told it's easier to cast longer rods because of the leverage factor but I don't find that to be true, necessarily. I own two switch rods and four other two-handers in varying weights and lengths, each serving a specific purpose.
What's your budget? Temple Fork builds a nice switch in varying weights as do other companies. Then you have to think about a line to match up properly for your rod and type of fishing, and then a reel that will balance well with the rod and hold the line and backing that you choose. It's not that complicated but you do have to define what you want to do in order to not make costly mistakes. Make no mistake. Fly fishing can be a very expensive endeavor compared to other types of fishing.