I remember some strategy discussions about this very issue in Civ5 BNW, as well as G&K and Vanilla. Although there's clearly a penalty for spamming cities beyond 4, more than in any other edition of Civ, players who practiced at playing "wide" found that you could in fact achieve higher scores and quicker victories. You had to use courthouses. And to take full advantage of going wide, you wanted to maximize your science output in as many cities as possible, usually with public schools at least. Wide games were very playable or Domination or SV; not so sure about other win types though likely Diplomacy as well.