It's possible that some of it may depend on playstyle, but I've found that I often do have reason to split them up. It depends on just what is going on at the time.
It seems to become more and more necessary as you expand the available map size. Early on, there's not much reason to. But after a bunch of turns, when you've got a bigger map to deal with and more buildings and more things that need doing, splitting them up can save time. For example if I've got a barrier of those evil buildings to pass through on one end of the map, and an important building to fix on the other, and my guys are in the middle, splitting them up can mean taking less total turns to get both things done. If I split them up, it might mean 1 turn having the combat guys go towards the wall and stomping it, and the next turn the others zoom over to fix the broken building; but if I'd had them all clumped up, I'd have to have at least one turn just using the rally command to move them towards the other objective after the first was done, so that 2 turns might become 3. Not to mention there are times when you might wanna set someone working on a tile, like on farmland or whatever, to improve it's production, and then have the others run about doing whatever else needs to be done.
You do have to watch out for Demonaica, but I find it's not THAT hard keeping out of his path; the real challenge is dealing with the destruction he causes. Dont forget that one command can affect potentially all of your current survivors at once depending on the situation; the rally command can allow you to move 2 or 3 seperate groups at once and clump them back together into one, AND can move each of them a significant distance, so it's often easy to get them back together fast if need be, and move multiple groups out of the overlord's way all in one turn.
The strategic part of the game definitely does seem pretty simple in the early turns, but it definitely ramps up quite a bit as you go along. Gets interesting fast, I think.
