The transition may be a a bit of a hassle depending on your riding style. Then again, I know many people with multi-speed bikes who found the gear they like and haven’t shifted in years, effectively making them single-speed bikes.
Some of them have brakes while others lack brakes, and “fixies” don’t free-wheel either; your legs are always moving unless the bike is at a dead stop. The last time I tried to stop suddenly on a fixie was embarrassing, and going down a hill was outright dangerous. I am with @jaytkay; leave brakeless bikes on the track.
Terrain enters into the equation, and since there is no single speed that can handle cruising on pavement, climbing a steep hill, or tackling inches-deep mud, I pretty much need shifters and three widely-spaced chain rings. Back when I was stationed in Orlando, there was nothing but level pavement so gears were less important for me then.
The only benefits I see are those inherent in a lower parts count; less maintenance, a little less weight (about a pound) and there is nothing to bend/break if you lay them down on their side. The downsides… I see too many to ever want one for myself, but that has to do with where/how I ride.