For perspective, I play exclusively marathon games on emperor (I should just put that in my sig but I'm not sure if I'm going to post much

):
Im not a big fan of nationhood, but thats probably because I play mostly domination games. When playing for domination I normally want to pop-rush things in the starving cities I just took over, not in my home cities. I also usually whip courts and theaters, not units, and I put off emancipation as long as possible.
I usually employ a professional army to minimize the costs of wars. My military cities either work on increasing the size of that army so they can constantly attack or divide into two+ forces, or building competent defenders. Pop rushing horsehockey units is just too expensive, IMO.
In terms of your economy, for a small nation (or when building a critical wonder in your capital) Bureaucracy is going to be better with 50% hammers and 50% commerce than the +25% espionage and +2 happiness from barracks, unless that happiness is really going to make the difference between having an extra couple population. Given that the techs you will soon discover will obsolete whales, ivory, and fur, this may occasionally be the case.
Offensively vassalage is better than nationhood, though ensuring a tech lead through bureaucracy or free speech seems better to me most of the time. For large economies with an average or above average number of towns, free speech is better.
The extra 25% espionage may be useful if a neighbor has a tech lead on you, but with the bonuses from jails and intelligence agencies, it becomes pretty small-time like the pathetic +10% research from free religion.
The best context I can think of is the synergy between pacifism and nationhood. There's the benefit of employing spies that are now better in output of espionage and gp points. You also want to minimize your military with pacifism, but you dont want to be too vulnerable to invasion hence nationhood. When you unexpectedly have visitors you can use the units from nationhood immediately as opposed to the next turn.
For a cultural or space race victory you may want to maximize your great artists/scientists with nationhood/pacifism, but for cultural victories you have to give up free speech for nationhood. This is probably not worth it.
Nationhood can be useful but not in most games I play.