Tactically that does not make sense. A unit, would stay together and use a path based on the most slowest / least mobile element of the unit. In military operations a unit, say a battalion task force will be giving one movement route and mission. At the military echelon which fights using battalions, the entire battalion task force is managed as one unit. A battalion task force is made up of company task forces, and each company task force will have it's own route and mission to support the battalion task force and is managed by different echelon of command. I can go on, but for the most part, biters are animals, with some sort of herd mentality.
In order do what you propose, to have half the unit take one route and a second half of the unit take a second route would require to split the unit into two units, designate a second assembly area, calculate the pathing for both new units, move the two units, then merge the two units back into into one unit. The other way to program that is to have some sub-units making up a unit. That would require some sort of recursive code incorporated into biter movement algorithm.
The other question then is how far do we go down this rabbit hole, do we include over-watch and bounding over watch-tactical movement techniques for units and sub units. What about phase lines, multiple assembly areas, alternate assembly area, axis of advance and attack, different operational techniques, such as flanking verses envelopment vs frontal attacks, faints, screening attacks and so on.
All of that would be much more CPU intensive, and require program and testing limited resources. This functionality would make for a good be a mod, or a DLC component in the future. Right now, Factorio manages biters, either as individuals are as a unit, but I don't know if it has the capability of having units within units.
I do agree with you that there is room for improvement and enhancement, but for release 1.0 I am fine with what is proposed in the this FF, especially since this focus of this game is design and construction, verses combat.