I don't think we'll agree on this but off-duty time is very important, imo. The 1/3 off-duty includes the soldiers who are sleeping, hospitalized (temporary), or dead. And yes there is more micro with 20x3 dwarf squads versus 6x10 dwarf.
I have a feeling though that it is play style that is the main difference. I use dwarves to patrol the surface and kill ambushes as well as guarding essential areas such as cavern entrances, surface entrance, and meeting zone. If you rely on traps to manage your entrance, you won't need those burrows guarded. But for me, if a squad falls below six dwarves then something will likely be under-defended. And setting a whole squad inactive because one died and one in the hospital will certainly lead to an under-defended fort.
If you only use your squads for kill orders, it doesn't matter how full they are and off-time becomes less important.
I disagree that it is a waste to have a professional military on patrol. That is exactly what a professional military should be doing (finding and killing hostiles). A novice military should be training more than a professional one. A highly skilled military that does nothing is a waste. And sending in the skilled military after the siege already broke on the trap corridor and is running away is also a waste.. the siege was already defeated. Dealing with FBs, giants, and titans usually only takes one decent squad too (entrance guards or the patrol deals with it automatically).