I have lots of dwarfs in my fort and several statue gardens. There is usually a very large amount of work to be done, including masonry (making rock blocks), masonry (building walls), mining (for stone) and hauling (move stone to the masons). Nearly all the dwarfs have the masonry and stone hauling labours turned on.
In practice, they do not waste any time partying. At all. While the building is in progress, all the dwarfs (except nobles, FG, RG and military) go to work, constantly. They do not waste any time. When the building stops, they go back to partying.
When they do party, it helps them gain skills (and hence indirectly stats) and (I believe) helps them form friendships and romantic liaisons. Noble dwarfs with high social skills are (I believe) more effective at preventing unhappiness. Also, if a statue garden includes any particularly spiffing furniture the dwarfs will get happy thoughts, and if you have different types of good furniture (eg. if you dump all those artifact millstones, floodgates, hatches etc. into your statue gardens) they will get several happy thoughts.
Also, you can be fairly sure that your statue gardens will have at least one member of the Guard hanging around, so you can use them to position your defences. (assuming you value the presence of the Guard; I do, you may prefer not to have a Guard)
Aaanyway, reason I'm saying this is that I keep seeing people saying how awful parties are, and I just don't think that's the case. It may once have been true, but parties don't currently harm your dwarfs productivity.
I also don't believe that dwarfs having friends from partying has a serious effect on tantrum spirals. If anything it may (properly managed) have a positive effect, because your dwarfs will have social skills that can help reduce tantrums, they can get *lots* of happy thoughts from partying, and they will all be more resilient to injury (as they will have higher stats).
Thoughts?