If so, maybe 3 speeds might be enough. Granted, I'm not a super fast or athletic rider, but I've been quite satisfied with 3 speeds for my commutes.

Of course the pro's and con's are easy to guess, because the numbers don't lie. With less than a 2:1 overall gear range, you have to give something up, either ease of cranking up tall hills, or having useful power going downhill.

And, much as I love gear hubs, I have to agree with @noglider that it's hard to make sense of IGH's from cost or weight standpoint. Adding one more gear to a cluster involves a simple piece of flat metal. Adding one more gear to an IGH is a whole new design. And it seems like modern derailer systems have gotten pretty darn reliable, even those of modest price.